It's not a monopoly because if this other theoretical company which does not exist existed then there would be more than one option and it wouldn't be a monopoly.
There's Android which has even more market share, and several behemoths like Google, Samsung, Xiaomi making similar sets of devices, so not theoritical.
That's not an answer, it's a decree.
Others can make their own phones and headphones just for their own devices, did Apple stop them?