It's not about market share of phones, its about market share of browser engines. Apple controls 100% of the market on web browser engines on the iphone.
That's legally immaterial. Microsoft controls 100% of the OS APIs on Windows. The legal distinction is whether the consumer has an alternative. Microsoft got in trouble because Windows was literally 99% of the PC market. Apple has something like a 30% share of the smartphone market - not even close to a monopoly.
Well they do own the iPhone. Whether or not the "iPhone browser engine market" itself is a thing within the greater "mobile browser engine" market is kind of a grey area.