is this a bit rude? Or just me?

[iamsoshocked]

Just got home from work, and DD (19 home from Uni) and 2 friends are at the kitchen table. One friend is veggie, the other has allergy to egg.

I came in, said hello, nice to see you again etc, to which they just replied "Hi". Then I asked if anyone was staying for tea? And they both literally just said "no".
Not "no thank you", or anything, just "no".
I said, "ok, well you're both very welcome to stay in you'd like to." Which was met with silence.

So I walked off.

Rude? Or have I just had a bad day at work?

In French, “merci” can mean no thank you in certain contexts e.g when you are being offered something. Easy to get caught out if you actually want what is being offered!

Oh please just give it up.

Absolutely uncouth, and NOT normal.
They wouldn't be staying lots in my house and I wouldn't ever offer a meal again.
I would be telling your daughter she can be "just how she is" in someone else's space.
Disappointing that your own daughter doesn't realise just how rude her friends are. My children would know that.
Definitely worth a conversation as you certainly don't her showing herself up in a similar manner.
Don't doubt yourself OP

They might have meant 'No, your daughter has not invited us.' It might have been a tricky situation for them if they thought that she didn't want them to stay.

Cheeky sods. Good manners cost you nothing and take you far!

I came in, said hello, nice to see you again etc, to which they just replied "Hi". Then I asked if anyone was staying for tea? And they both literally just said "no".

That doesn’t sound like an invitation to me. They could have interpreted it as you meaning “oh, fuck, is that more mouths to feed then??” and/or felt it would have been presumptuous to interpret the question as an invitation.

Their lack of response to your next question was rude though.

This was my thoughts, especially if any of them are ND. As far as they are concerned they answered the question.

As for walking out and not saying thank you for having them stay, I've noticed as my childrens friends got older, living on their own at uni etc they stopped saying it.
They wouldn't be saying to each other at uni so maybe in their minds they've stayed at your dd's house, not yours?

If you are asking them whether they are staying, it means that you would have fed them. So a plain 'no' was rude. A 'no, thank you', would have been the correct response.

This.