Is this an accurate description of longtermism, or just Salon playing the internet media outrage game?
It's not about an ethical obligation to create digital people, more a way to quantify what's at stake if humanity were to be destroyed. The point that he makes is that most people use 8B deaths as the "cost" when doing cost benefit analysis around a hypothetical apocalypse. Bostrom thinks the cost is much higher since it should also include all humans that will ever exist. When you try to quantify how large that could possibly be, you run into the possibility of far-future digital humans and he includes those in his calculation.
The basic argument of a longtermist position is that, similar to how throughout history we extended the set of entities we give moral value to (some humans -> all humans -> all humans + non-human animals -> ...), we ought to further extend that set to include future (i.e. not yet born) humans.
I don't personally find the argument very convincing, but it's not as absurd on its face as Salon is trying to paint it.
I do find it interesting that most people don't object to the idea of thinking about what is best for your hypothetical grandchildren.
I agree, my objection to the serious (say, Bostrom's) version of the argument is exactly along that line. When we add new entities to our set of those worthy of moral consideration, we don't imply they should all be given equal value. In less abstract terms, it's perfectly consistent to hold the following two moral positions: "animals should not be mistreated gratuitously", and "it is morally permissible to eat meat"; this would amount to having an utility function where animals have some nonzero weight, but not enough weight to offset reasonable human desires.
Likewise, that averting existential risks should trump all other considerations would not only require that future humans have some nonzero moral value, but that their moral value ought to be high enough that everything else pales in comparison.
Something like "my concern with future humans decreases exponentially with time", like you propose, still leaves us with a "normal" position: "sure, we should care somewhat about the future of mankind, but not, like, above all else". Both the premise and the conclusion of this "boring" position seem much more reasonable to me.
Presenting a contested implication of X as a fact about X is basically lying. For example, suppose I observe you advocating for solar panels in a city meeting. I know that solar panels are often manufactured by slaves in Xinjiang. Is it appropriate for me to describe you as an advocate for slavery in Xinjiang? Obviously not - that would be a lie.
The reason Salon doesn't write "universal human rights are a quasi-religious worldview, influenced by the bloodstained French revolution and the early 20th century communist movement" is because Salon actually likes human rights.
The hits on Robin Hanson were particularly bad. If you didn't read further you would think they were advocating slavery on the basis of sex and race.