Guidelines are not rules.
Certain factors are more likely to get the game into the review pile, that doesn't mean that if you meet factor A then it will pass B.
Maybe you can point out my mistake, but I don't remember reading anywhere where Itch says that a game is indexed instantly if it meets the proposed guideline.
The actual process has never been explained by the staff, we can only speculate based on what was observed.
As you say, there are games of very low quality that are indexed and others that clearly have work done by the developer and that are not indexed and we really don't know the exact reason.
By the way, you ask that they not tell you that there are thousands of games on Itch, but at the same time you say that there is a lack of staff. Don't you see the relationship?
If there is only one person reviewing and 10 games are uploaded one day, that person can review all 10.
If 10,000 games are uploaded tomorrow, that same person cannot review them all.
How do you solve the problem?
Possibly an algorithm that rates certain risk values, it is possible that this algorithm includes random factors, something very common in QA processes.
Furthermore, it has been said many times, do not depend on being indexed as a means of advertising.