The colloquial use of the word “autism” carries with it a specific connotation and mind image. That primarily negative stereotype is being reinforced by the joke by way of it being delivered as a medical diagnosis (“the results are back”).

Your parent comment is arguing against perpetuating the wrong negative connotations and lack of understanding of autism.

Not to say the original author was doing it maliciously, I don’t think they were.

And it's time that colloquial term is put to a rest. We've left other terms behind us which used to be thrown around mindlessly but were (are) actually hurtful, we will manage with that term too.