More evidence they are autistic? And maybe only children.
Even if a kid is a literal friendless, they usually get invited to a sleepover or experience one that siblings host or get invited to, especially if they are the younger.
Before I get ree-ed at, a lot of autistics don’t go to sleepovers because they have complex personal rules on bedtime or sleeping arrangements.
So it’s not that they don’t get invited, it is that they turn down the invitation.
Personally I didn’t sleepover at local friends places. It was easier just to go home, but I experienced many sleepovers at friends who lived elsewhere.