The wording given isn’t exactly accurate. It’s not that other games have “stolen” your Pokestop, but that your spot is occluded by proximity to other Pokestops. Here’s a thorough explanation of how things work: Why an accepted Wayspot may not be used in Pokemon Go (S2 cells and Pokemon Go)
So, there’s likely another Pokestop within the same L17 S2 cell, and that’s why your nomination failed to become a Pokestop.