I don't think that's right, though I deplore the ignorant subhumans who were downvoting you like some kind of fucking spammer.
> 1. You can't be a marine organism, because developing advanced technology requires combustion, and water isn't something you want to be carting into space with you.
Humans cart water into space with them; an 80kg human contains about 50kg of water. It hardly matters whether that water is all inside your skin or partly outside your skin but inside your spacesuit.
The combustion question is more interesting: can you develop advanced technology without being able to burn things in your native environment? I think there are three reasons that the answer is "yes":
A. There are lots of other ways to manipulate material properties other than changing the material's temperature. The humans use fire because it's easy, but that doesn't mean it's the only way. Electrolysis can be rather surprisingly effective, and there's also manipulation of pressure rather than temperature, which is how the humans finally produced a room-temperature superconductor a couple of years ago, and of course crucial to the Haber-Bosch process. Manipulating pressure is very hard in a gaseous environment; being underwater makes it a lot easier, especially if, like a sperm whale, you can dive from 1 atmosphere down to 2 km of depth (200 atmospheres), or if, like a mantis shrimp, you can create cavitation bubbles that create plasma hotter than the surface of the sun when they collapse. Organisms like electric eels might be able to do electrolysis intrinsically, without tools, if they were intelligent, but you can also pound native copper into wires to make windings for electrical machines like generators.
B. The humans do a lot of material manipulations in environments they can't live in: the hard vacuum of an electron microscope, the interior of a furnace itself, the freezer in your house, the sulfuric acid of a car battery, the violently water-reactive environment of a Grignard reaction, the boiling airless interior of a soup pot, even the room-temperature airless interior of a water bath. Intelligent marine organisms could do their hot material processing in a trapped bubble of air as easily as the humans do their solvated material processing in a trapped flask of water.
C. A fish is a far more complex, delicately balanced, and robust system than an Audi, and yet genes are able to construct a fish entirely underwater, without using any fire. If genes can do it, obviously it can be done, even if the humans haven't had to figure out how to do it themselves.
I do think doing without fire in your native environment on Earth would be a lot more difficult, though.
But what about environments that are radically different from Earth? Venus's atmosphere at the surface consists of mostly supercritical carbon dioxide, where the familiar solid/liquid distinction ceases to make sense, and it's warm enough for phosphoric acid to polymerize into polyphosphates that are soft and yielding like carbon chains are on Earth. On Venus's surface, carbon chains are unstable because the temperature is 500 degrees, much as many compounds that are stable in dry ice are unstable at Earth's surface temperature. So think about Titan's atmosphere, which is mostly nitrogen with methane in liquid, solid, and gaseous forms at the surface, at a temperature of 94 K. Think about what it's like inside the sulfur volcanoes of Io, or the water ocean under Enceladus's shell of ice, or the catastrophically periodic environment on Halley's Comet, with ice exposed to vacuum most of the time.
> 2. You need a musculoskeletal system which can support a large brain. For non-marine organisms, this implies something with a big head on top of an upright stance.
You might not need a large brain at all; Alex the African grey parrot had a brain the size of a walnut and was capable of answering questions like "Which color bigger?" or "How many green cubes?" --- up to three or four, anyway. Corvids, about the same size, construct and use tools. If you do need a large brain, you could still be aquatic, or the brain could be located in the middle of your body instead of at one end as it is for octopodes (who can walk around on land without a skeletal system at all, just not breathe there the way some other mollusks like land snails do), or distributed in smaller brains around your body as it is for corporations or anthills.
Where does the "upright stance" thing come from? I don't see that you've made an argument for that at all.
Organisms floating around the atmosphere of a gas giant don't have to worry about supporting anything, just like marine organisms. And there are many other strange places self-reproducing patterns could arise and become intelligent: the magnetic fields and convection cells of a star, the nuclear reactions in a neutron star, the plasma winds of a nebula, the deep subsurface which on Earth is largely populated by ferric-ion-reducing archaea.
> 3. Having a surplus of limbs is contraindicated for large organisms. Large limbs are expensive to build, and larger organisms are more constrained by gravity than surface tension, which makes multiple limbs less useful.
Yet humans have four limbs and only need two or three, and giant squids reach 12 meters in length and hundreds of kilograms in weight while having ten tentacles and arms. (The colossal squid weighs more but is a bit shorter.) Pando is a much larger organism than any human, and has thousands of "arms"; they're just called "trunks". Coconut crabs, who can live their entire life on land (as adults they lose their gills and can no longer survive underwater), weigh 4 kg, about ten times bigger than an African grey parrot or crow (300-600 g); their ten legs (including their claws) span over 90 cm.
> 4. You need appendages for manipulating tools, and voices to debate and share ideas.
You probably need some kind of way to manipulate tools, but you could use the sucker-covered arm of an octopus or starfish, the beak of a crow (obviously enough), the claw of a crab, the trunk of an elephant, the ephemeral pseudopod of a slime mold, or the tails dolphins use to send one toroidal vortex bubble through the middle of another. There's no reason the appendages have to look much like human hands; even if they do, they could be attached somewhere else, like the hind feet of chimpanzees or the trunks of elephants. If you can reflect or refract light, you might be able to manipulate tools by focusing light. You probably need some way to communicate, but it doesn't have to be a voice; octopuses can create moving patterns on their skin that other octopuses can see by expanding and contracting their chromophores, and Deaf humans constantly debate and share ideas without using voices. Most fish can sense electrical fields in the water, and some fish like the electric eels mentioned above can generate them as well.
And of course if you have manipulating appendages of any kind, you can manipulate things in the world into symbols that others can observe, as I am doing now with my manipulating appendages.
> 5. To develop non-trivial intelligence, you need to be multicellular, and multicellular organisms need to be able to reproduce. So, there is probably going to be sex involved, along with all the complexity that goes along with that, such as sexual competition and selection, caring for the young, and all the politics that arise from these kinds of interactions.
Slime molds aren't multicellular, but they can learn to run mazes, but the whole multicellular thing is a red herring; organisms are defined in part by being able to reproduce, and slime molds, as well as many other single-celled organisms including the remarkable foraminifera, have sex. (Slime molds have many more than two sexes.) Moreover, many organisms reproduce without sex, and many organisms that have sex don't care for the young. So I think your argument in this point is so weak as to be incoherent.
Overall, I think you've entirely failed to make the argument for bipedalism.