I don't think that makes sense.

I'm not disagreeing with your overall point, but autistic in that comment is attempting to capture that "literal rhetoric but socially unaware" style of argument people can have when they argue a point too black and white.

"Retarded" does not imply this

I would say the problem with the comment is more in line with how OCD or adhd is used, improperly. Like despite autism being a spectrum with traits that can't be reduced to that one stereotype.

It's being used to invoke a stereotype of neurodivergence. Not as a stand in for general stupidity.