It’s like a dunning Kruger effect.
It is also important to note that good does not necessarily mean correct or accurate (even if that certainly can be a part of it)it means useful for guiding proper behavior.
With that said, the hypothesis is not that bad mentalization abilities is equal to autism. It is that bad mentalization causes you to adopt/learn unconscious behavioral patterns and those patterns are what we classify as autism. This means that even when you “fix” the bad mentalization ability. The person still has all of those behavioral patterns ingrained in them.