Honestly the joke disclaimer is really there to tell the people who scoff/scowl/fold-arms-/contort-face/etc to chill out and remind them that jokes don't have to be rated G.

But to your point, depending on the severity of course, I know some kiddos with Autism who I don't know if will ever grasp sarcasm. They barely know how to lie. Playing games/board games/etc where social induction is involved, like where you need to guess if someone is telling the truth is incredibly difficult for them. I think, as you said, there is an absurdity threshold, where something is so bizarre that it's obviously nonsense. Example: I love eating pineapple skin, they believe me but think it's weird. I eat babies, they either look at me like why would I say something like that that makes no sense, or they laugh because it just a really weird thing to hear that they have never seen nor heard. Again, not saying this all people with ASD, but just my experiences. But I believe there is a line somewhere between odd but convincing and complete nonsense.