I'm pretty serious with this argument, though. The entire point of the post here is that no idea is too odious to be considered unworthy of polite debate. Life extension? Sure! Gay people are evil? Sure! (For some reason, and against all evidence, we hope nobody will be convinced of that.) Eugenics is great? Sure! People of a certain race are genetically inferior and unworthy of human rights? Sure! Those who are apostate from my religion should be killed? Sure! This article advocates for literally no bounds on what should be considered an idea worthy of protection, because who knows when a "wacky" idea might be secretly good. I don't believe I'm exaggerating one bit from what the article says. (I might be exaggerating from what the author means, but only because we're supposed to self-censor some of those ideas, and the article is unclear on which ones remain worthy of self-censorship.)

So why are we treating "This person is morally bankrupt" as the sort of argument that is unworthy of polite debate? What fundamentally makes it a worse idea than any of the above?

I think there's an incompleteness problem here, yes - but I think that problem genuinely applies to the original argument. I'm just pointing it out. (And the gist of The Abolition of Man, which I mentioned in another comment, is essentially an incompleteness argument against the idea that you can have an ethical society evaluate its own ethics without a base moral code.)