User, Professor,
"...@autumnal 'you're welcome' - why do people write this on mumsnet? Welcome for what? No one said thank you to you."
"...It's a really dickish way of saying, "I'm right and that's that", particularly when they aren't right."
It was a (clearly misguided) attempt at humour. 
Only aimed at a tiny handful of posters (not at the majority, including OP, who are just bantering) seriously, patronisingly, insistent that only their preferred term, be it "dinner" or "supper", was acceptable.
I was sticking up for any "tea" users.
Fine to have a preferred term; not fine to dictate and assume you know best. The "you're welcome" was meant to underline their unasked-for assumption, that's all.
[As mentioned earlier, depending on context, I use all 3 terms, so have no axe to grind either way...well, apart from "kitchen supps"/ "country supps", which is a step too far for anybody].
It obviously failed. I'll go and have a stern word with myself.