For contrast, if you say "90% of the planet's wealth is held by the upper 1%," and your discussion partner asks you why you support the monster Stalin, I think you're on solid ground asking them WTF.

Maybe. It could have been your discussion partner's experience that everyone who brings up the 90% thing has, in fact, been a communist. If that's been their experience, then based on the knowledge that they have, that can be a reasonable question to ask. Compare with the claim "Marx wrote that [whatever]"; even though this might be a neutral factual claim in principle, in practice anyone who brings that up in a discussion is much more likely to be a Marxist than someone who doesn't.