Actually, I could imagine you reading that comment and feeling it still misses your point that 0.999... is undefined or has different definitions or senses in amateur discussions. In that case, I would point to the idea that one can makes propositions about a primitive concept that turn out to be false about the mature form of it. One could make claims about evidence, causality, free will, knowledge, numbers, gravity, light, etc. that would be true under one primitive sense and false under another. Then minutes or days or month or years or centuries or millennia later it turns out that the claims were false about the correct definition.

It would be a sin of rationality to assume that, since there was a controversy over definitions, and some definitions proved the claim and some disproved it, that no side was more right than another. One should study examples of where people made correct claims about fuzzy concepts, to see what we might learn in our own lives about how these things resolve. Were there hints that the people who turned out to be incorrect ignored? Did they fail to notice their confusion? Telltale features of the problem that favoured a different interpretation? etc.