There's a huge difference, though. Creating a nuclear weapon encourages others to do the same. An AGI, if done right, would never allow the creation of a second one with conflicting values; there should be no second AGI.
I'd tend to think the reverse: the idea that we can recreate general intelligence requires some degree of speculation, but once we have general intelligence running on a computer, it takes a fairly contrived set of conditions for it to not scale up exponentially.
But let's change the question up a bit... There are billions and billions of flying intelligences on this planet. Birds, insects, even mammals. Nature has already created that. We've created things that are even better at flying fast and carrying more weight. So simply looking at 'flying cars' and saying they didn't happen so AI can't happen is at the least, very ignorant.
If nature can create something randomly, we can create something directed in a shorter period of time (well, we don't really have another 4 billion years to try). AGI is an eventuality.
We have evidence of intelligent species (ourselves) invading. We also have evidence spaceflight is possible.
An alien invasion is speculative at this point because there is no alien species on to which we can base any scientific refutation. It is not impossible, it is simply impossible to define any probably of occurrence.
"if done right" glosses over quite a bit, yes.