It sounded like that was based partly on Supply (Army size). But the AI only knows it's own Army Size, and not the human's army size.
So how can the AI's outcome prediction be so accurate? At game 3 against Mana, the AI's outcome prediction changed from 60% win to 99% win, before the AI decide to go up the ramp. It had no way to know if the human had more army up that ramp
You can see this in the replay-- at seemingly random times, its outcome prediction jumps up, even when it hasn't had any interaction with its opponent. But that's precisely why it's going up-- it notices that its opponent has not executed a faster rush or cheese that it's unprepared for, hasn't expanded early, and the scout has not been destroyed.
Similarly, after having won a fight, the worst thing that could happen is that a bigger force emerge from your opponents base, destroying your army and giving them a chance to rebuild. When that does not happen, you know that your opponent probably doesn't have such an army (because otherwise they are falling behind in resources due to being bottled up). Either way, after a certain point it's worthwhile to press on to cause more damage, because you're now far enough ahead in resources that you will win regardless if they manage to repel that particular attack.
Is the outcome prediction score, itself, also produced by AI training?
It has to be, right? Because it's clearly not just calculating the outcome based on Army Size of the AI and the human. There must be some non-direct way it's calculating the outcome prediction.
I mean, the hard part is accurate outcome prediction. Once you have that, it's easy to train an AI by just throwing CPU's at the problem and making the AI play a crazy lots of games