No, that's not "almost tautological". In fact, it is a form of the appeal to tradition fallacy. (LIt is tautological -- as are many applications of that fallacy -- if you take the assumptions underpinning rational choice theory as axioms, but it stops being such once you drop the perfect information assumption.)
Your reframing of it in the next sentence subtly acknowledges this, when you shift from "if there was enough profit" to "there’s not enough estimated profit", despite that you are responding to a post that explicitly is arguing that the estimate made of the profit is dubious.
Those aren't logical fallacies; they're just not very useful.
The GP didn't say that.
As for which informal fallacy the GGP's argument contained, I would say it was more like a false dilemma. And the GGP did say "almost tautological", which implicitly recognizes that there might be other possibilities.
> people often express this criticism by saying "your argument is tautological,"
I have never seen this (nonsensical) way of expressing the criticism that an argument is false by saying that it is tautological (and therefore true).
If you wish to see usages like I described, have a look here.
https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
Second, it’s almost tautological. If there was enough profit to justify the development time and cost, drug companies would pursue a vaccine. Since they’re not pursuing a vaccine, you can assume there’s not enough estimated profit compared to other research ventures.
The second point includes considerations of difficulty. There are many strep strains. Strep can trigger autoimmune diseases. Will the costs of surmounting these challenges be rewarded above other pursuits?