I think you've missed the point here. Neither Apple nor Epic have enough market share in their primary market (smartphones/games) to be considered monopolists.

So instead, people move the goalposts to an aftermarket of the product (iOS apps) and claim that Apple has a monopoly over that market instead. But every company has a natural monopoly over their own products, and by the very same logic you could claim Epic has a monopoly over the aftermarket of their product (Fortnite skins).