Sorry, that's not the case. Anyway, here's a synopsis of the decision which sets a bar for the courts consider monopolistic practices from Wikipedia:
Market share of the total user space isn't necessary for there to be a monopoly. This goes back to the company town analogy where a business runs a closed system that includes rent, utilities, services, and pay for its employees. You don't have to own a majority of towns in America to run an illegal monopoly. You can run a town with 10 people and still create an illegal system.
"The plaintiffs alleged that Microsoft had abused monopoly power on Intel-based personal computers." Apple owns an even bigger share of IOS-based phones; a complete share.
It's not about alternative platforms, and it's not about Epic's other practices.
If it was a monopoly when Microsoft made it hard to use IE alternatives, then it's a monopoly when Apple makes it nearly impossible to use third-party software of any kind without approval.
But there is a key difference you are missing in the Microsoft comparison.