The reason I posted publically was because it could have been almost anyone who made the same mistake, so it was useful to make the correction publically. I didn't expect to have such a strong reaction - I apologise if I offended anyone, as that was not my intention, but I won't apologise for the statement itself because what I said was correct and not offensive.
Westrim, citing sources is viewed very positively on this forum and in certain sections is strongly encouraged.
You're correct that the term "trans" can be a noun, I should have said "trans isn't a noun in this context". When it is being used as a short-form of "transgender" then it is an adjective, because "transgender" is an adjective, when being used to describe people.
It's not about whether certain words can ever be used in a different way - in that specific context, that specific word was playing a specific role as an adjective and was used incorrectly.
I also agree that there can be a point where someone taking offence is ridiculous. But this is not one of those instances. Reducing someone to the prefix describing an aspect of their gender identity is offensive. Try and see it from a different perspective - I am a woman and I happen to have quite large breasts. In a context where such a statement would be relevant, I wouldn't be offended at being referred to as a "large-breasted woman". But if someone referred to me as "a large-breasted" without the word woman, I'd feel that was rude. As Paul said, it would be equally correct to remove the article and just say "large-breasted" or "trans".
Honestly, it was only partly about the actual content of the word (in that when there's the possibility of a mistake offending someone, I think it's more important to point it out than if it was a simple typo which no one would be upset by). The overall point was one of grammar; I teach english grammar, it's something I care about.
I just looked a bit more into the term "trans fat" and to see if I can find any instances of trans being a noun. Trans fat is so called because the fatty acids are "trans isomer", meaning "between" (I don't know what an isomer is, but that word itself is a noun - trans is describing it, and the two words together form what I call a "noun phrase" but I don't know if there's an academic term for the concept). Trans is just latin for "between", which isn't a noun - it's strictly a preposition, but it serves a descriptive purpose.