Delaying kindergarten by a year by doing another year of preschool, and delaying your HS freshman year by a year to do nothing but train and play video games, are apples and oranges.
Delaying kindergarten is very common and there is nothing illegal about it, although it is an example of privilege.
i know repeating 8th grade for athletes is somewhat common, but I was not aware it was common for kids to do nothing but train and play video games during that year. In my area the kids that repeated 8th grade usually went to private school though (another example of privilege I guess), because I imagine the public schools weren’t on board with holding back a kid that was ready for HS.