Why should the presence of replies to your comment mean that they don't agree with you? Sometimes replies add more context or are simply elaborated me-toos.

I think that's the exact point the parent commenter is trying to make. One person said Google has wanted to get rid of XSLT for years, which someone else responded to saying that it was Mozilla who made this proposal, and then someone else responded with details about Google's history of wanting to get rid of it previously, at which point the question around what part of the second comment been untrue entered the picture. That question doesn't make a ton of sense, because if the reply to the second comment was assumed to be disagreement, the same logic would presumably apply to the second comment itself, so the question "what part is untrue?" would be just as valid to ask about the "disagreement" with the original statement that Google has wanted to get rid of XSLT for years.

Of course, I could be making the same mistake in reading your comment as expressing disagreement with the one you're responding to! If that's not the case, then I'll happily accept my mistake so the chain can hopefully stop here.

(edit: I'm realizing now that this definitely is my own mistake, as I misread which comment this one was replying to. I might need to invest time in finding a new app for reading HN on mobile, since the indentation levels on the one I've been using clearly aren't large enough for my terrible eyesight...)

This is a good comment and there is an answer to your question. I know because I react the same way sometimes. It's about insecurity. They're interpreting it as negative by default either because they've been treated poorly before or because they have low self-esteem. Either way, their state shapes their expectations.