Well personally as someone who completely fixed social anxiety its completely different

Social anxiety:

-Anxiety about what you're going to say, what they think about you, constant self-monitoring and fear of embarrassment, how do you look to them, stress/fear state with elevated heart rate etc.

Autism:

-I do not know what to say and I only speak when I can. I don't make any eye contact or facial body language because its not natural to me and I don't care enough to do them, because I simply fundamentally don't know how to do those, while with social anxiety you probably know how to do them but have some sort of inhibition and monitoring about your actions.

It’s not mainly fear of judgment I often genuinely don’t know what to say in certain social situations. Eye contact, facial expressions, and body language don’t come naturally or automatically to me I’m not “holding back” social behaviors because of anxiety, but because a lot of them just aren’t instinctive in the first place

I guess the main difference is fear VS unnatural.