>The answer is right in the text you quoted. Because it's a different market than the smartphone itself and unfair competition.*
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?
Well yes, that’s what laws are?
That (a kind of decree) is what any law is, including unfair laws or Jim Crow.
Doesn't mean it's an answer to the issue why that should be the case.
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.