A clarification since perhaps I didn't know how to explain myself, I don't mean that every game must be reviewed manually by a human.
I mean that every game must be approved (manually or automatically) before being indexed. In the absence of exact data on the process, as you say, it is most likely that they have some program that makes these approvals automatically and under certain conditions, throws the game into a manual review process, as you suggest.
However, this automatic process is precisely a review process; if it did not exist, every game would be indexed as soon as it becomes public and we know that does not happen.
If a given game has not been indexed shortly after being published, it will surely enter the queue to be reviewed by a human.