That sentence wasn't an argument. The two paragraphs containing the sentence do constitute an argument or something like one; they are not "fully general" in any sense that seems problematic to me. The most one can say is this: they claim that if a proposition is socially unacceptable to state then it's less likely to be true. I'm happy to stand by that: I think "unacceptable" propositions are less often true than "acceptable" ones. Do you really disagree with that?

Incidentally, I wasn't primarily thinking of X as being a proposition but as a behaviour or an attitude. I bet that among, say, politicians, advertisers, tobacco company executives, television evangelists, there are common habits or ways of thinking that "of course we wouldn't mention in public -- they wouldn't understand". And that neither you nor I would be keen to defend those habits or ways of thinking, even if we're pretty sure we do understand them.

For the avoidance of doubt, let me repeat something I already said. Of course, some "unacceptable" ideas, behaviours and attitudes are in fact perfectly sensible and are unacceptable only because of silly social traditions or whatever. I claim only that such unacceptability is a useful warning signal.