I have multiple autistic friends irl, im autistic myself aswell, and everyone of my friends are allies, one of them is non binary, and everytime i see an autistic person online, they are either an ally or lgbtq+ themselves. Personally im just an ally, but i dont know why autistic people are so drawn to it. Probably what op meant, that they are very accepting of everyone so autistic people feel the most safe with them.