This is the concept that's about to be tested in court. The majority of the revenue generating segment of the smartphone market is iOS while the majority of the overall smartphone market is Android. I get the sense that Apple did this on purpose but the Unity lawsuit may write a new chapter on what it means to be a monopoly. This Planet Money episode talks about this dilemma: https://www.npr.org/2020/09/18/914563075/apple-v-everybody
I'm going from memory, but I have looked into the actual definition of a monopoly in the context of anti-trust. If I recall correctly, it doesn't even explicitly say that you need a market majority. It basically says that if you have undue market influence and power and use it then you have a monopoly. A market majority is simply one of the most common ways that that is attained. Again, that is from memory. I'd have to search for the language again to be sure.
Like you point out though with it being tested in court, maybe precedence is strong. Those laws and definitions were written a long time ago. Today's companies like Google, Apple, Facebook, and Amazon are very different from companies of yesteryear.
Apple is probably the biggest bully of a company in the world. They absolutely don't just make more money. They use their power to ensure they make more money.
