This was my takeaway as well - theory of mind can sometimes be described as 1. "literally does not understand that other people have their own thoughts" or 2. "cannot reliably predict what other minds would think". Most high-functioning autistic people I've met clearly don't meet the first definition, but definitely meet the second.
The second definition makes more sense to me since it also comes up in non-autism diagnoses, like personality disorders. They have trouble reliably predicting what other minds would think, but for different reasons.
High-functioning autistic people would meet the 2nd definition when trying to predict what neurotypical people would think. But they're also probably better at predicting what other high-functioning autists would think than neurotypical people are. It's just easier to imagine what someone might think if they think like you.
A part of all psychology diagnoses is that it must both match a definition and negatively impact someone's life. So if you never had to care about what neurotypical people think, then it wouldn't rise to the level of a problem and there's no diagnosis.
You see people care about this stuff because they go to their therapist and say, "boy there are a lot of neurotypical people I have to predict, and being slightly better at reading autistic people doesn't make up for it."
Unreliability is relative. Everyone botches this up all the time. The most neurotypical person I ever met literally questioned the whole premise of predicting what other people were thinking and said that most people will just tell you things if you ask.
> most people will just tell you things if you ask
In my experience people are often bad at explaining their inner emotions. Rationalisation is one of my favorite words!
I will often ask a friend why they acted a certain way, and I will carefully listen to what they say, but I often disagree with their stated motives - denial and lack of self-awareness are rife. This has nothing to do with people consciously trying to deceive me (there's another topic!). I have the same faults. I'm average at reading people, yet all too often I have to trust my own flawed instincts about what I think is going on.
The second definition makes more sense to me since it also comes up in non-autism diagnoses, like personality disorders. They have trouble reliably predicting what other minds would think, but for different reasons.