Is this rude?

Yeah, I'd say that's kind of a dick move. As someone who is battling a chronic illness, it's just not always a possibility, ya know? Sometimes I don't feel well enough to go out, or something I'm just tired of being stared at. So I would definitely be a bit miffed about it. xD

Not as bad as the friend who told me "Ya know, we all go through things." when my hip was broken for the fourth month or so and it was making me grumpy. 🙄 Like... yes, we all go through things. But sometimes they're more severe than others.

it's more rude to say you can't have fun because you have chronic illness

Yes. If someone told me that, I'd tell them to fuck off straight away.

I said to my mate, 'hey how are you?'
'I'm dying of cancer.' He replied. I wouldn't have started the conversation any other way. It's awkward.

are they aware of the illness? maybe they didn’t mean to be rude

Nope.

I think that ever speaking to someone who has chronic illness is rude. Just ignore them. 👌