it has nothing to do with discrimination, the way the rules are set up just make it difficult for someone using an easy creation middleware to pass review. there are a few standards that you need to manually code in to get past peer review, along with other things that most beginner developers wouldn't think about when making a game (title safe area, proper text sizes, content rules). using something like IGM suggests the person making the game doesn't know as much about programming, hence why they are using IGM to begin with, but then they will fail review because they don't have the knowledge needed to pass and fix their game.
for example, let's say you made a game in IGM with saving. you'd fail because you don't have a standard save set-up, because you need to be able to handle the user pulling out the memory unit multiple times while saving, which would crash the game. someone using IGM probably doesn't know how to do this, if they did, they probably wouldn't be using IGM to begin with (p.s. i heard IGM isn't great).
there is one game on xbox that uses indie game maker, but it was made by the people who programmed indie game maker. also, i haven't seen anyone beyond the program creators using IGM for xbox development.