First I’m not sure if he’s autistic or just saying “I’ve got Asperger’s so I’m allowed to be an asshole and you just need to deal.”

But let’s assume he is…

High functioning autism is not associated with involuntary Nazi salute gestures. It’s also not associated with the inability to learn the significance of the gesture.

So if autism is somehow related, then how would it be?

Well the “nice” option is he is going full 4-chan troll mode thinking it is hilarious without processing just how bad it is.

The other option is that he thought he did a credible cover to blow a dog whistle, but was unable to process that he was blowing a tuba instead.

In short, even if it was because of autism, it still almost certainly means it’s still quite on purpose, so it’s hardly an excuse that makes things any better.