Same answer, I suppose.
I take the point that this is complex, but that doesn't discard OS/community by default. It's not impossible that community built OS could produce software trustworthy enough to be widely usable. FOSS has played in high stakes arena successfully before.
I would say that the point you and GP make demonstrate that "move fast and break" or "MVP" stuff can't be applied easily. That's not specifically something that impacts OS. It's just a difficulty of the space.
That’s sort of the risk you take when using open source software in general. You can put in a PR to fix that edge case if you feel so inclined. In my experience the IRS can be surprisingly understanding about mistakes on your tax forms (ONCE you get a human on the line…).
The question is, do you think the potential penalty will cost more than the tax software? It comes down to managing risk, and some people may prefer to take the risk, especially if they have relatively simple tax situations.
It's easy to have open source project where the stakes are "bad translation" or something that doesn't result in fines or problems with IRS/law.