It’s really just a combination of timing, numbers, and mediocrity.
Firstly, there is suspicion and likelihood that men have won women’s medals in previous Olympics, but it was not well known and was in a lesser sport so no one noticed.
Second, look at the time line.
Rules changed.
A small number of mediocre male athletes are trans identifying. No elite male athlete is trans identifying.
An even smaller number decide to chance their arm in women’s sport, retrospectively.
Some start making progress.
Women push back. Some sports where males make progress change the rules.
Other sports either still have males in them who are yet to come to notice or bring about their sport’s rule changes.
It still remains to be seen what will eventuate at the Paris Olympics. If it wasn’t for swimming, cycling and athletics bringing about rule changes, there’s no doubt there would be an impact in those sports in Paris. So it will be other sports.
The IOC rule change opened it up for a gradual increase in the number making a prospective attempt by males - those at the start of their careers who could enter the elite pathway. This number would gradually increase over time if it was purely self ID and no pushback for individual sports to change their rules. It would be x number in Paris, then x + y in LA, then x + y + z in Brisbane.
These numbers would not be overall large as absolute numbers because the number of men who are trans identifying and physically elite enough to beat women is finite, but it would be enough over so many sports to make an impact, which in reality is just one male in women’s sport.