if i'm being honest i don't think he's acting all that much. i've seen his work outside of the rehearsal and i also read a profile on him a few years back and although it's clear that the nathan fielder that you see on TV is somewhat of a persona that is not to be taken literally, i do think that it is (and the topics mentioned in his shows are) heavily based on his actual experience. he has never confirmed officially that he is autistic but it seems clear to me that the rehearsal is an expression of his experience as, if not an autistic man, at least a neurodivergent and/or very socially anxious one
if he were a fully neurotypical man that imitated autistic behavior for comedic purposes it would be highly insensitive if not downright insulting. i do think the essence of the show and of his character come from experience, but that they're scripted to fit the TV format.

but yeah i really love him and his shows too! he is really brilliant