That is how the DoJ defines the market.

https://law.justia.com/cases/federal/appellate-courts/F3/253...

> The District Court determined that Microsoft had maintained a monopoly in the market for Intel compatible PC operating systems in violation

Change "Intel compatible" to "A series compatible" and it's Apple. Change it to "ARM compatible" and it's still Apple given they have 60% market share in the USA. Only countries enforce anti-trust, there is no world government to look at market share it the world.