If the products aren't interchangeable, then they are separate markets. Someone in the market for a part that fits a Ford can't solve their problem by buying the similar part that only fits a Chevy. Likewise, he can't but a new tire to replace a carburator. The tire and carburator are also in separate markets.

However, he can buy an aftermarket part. It's a different product but the same market. So they don't have a monopoly.

I don't know much about antitrust law so I can't comment on that But I do understand basic economics. And from an economic perspective, Apple most definitely created a monopoly which they most definitely use for rent seeking.