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.

Not very convincing.

There's Android which has even more market share, and several behemoths like Google, Samsung, Xiaomi making similar sets of devices, so not theoritical.

But that's not even close to the argument you made.

Also, you're confusing markets here. We're not talking about the smartphone market. We're talking about the market of apple accessories.

Apple solely controls that market and also participates in it. Its a true monopoly.