Claiming that Apple has a "monopoly" on app sales on its own devices might be technically true but is not a meaningful statement when there are plenty of alternatives to Apple devices on the market.
It is about as meaningful as claiming a gas station has a "monopoly" on gas sales in a three block radius, or that a bar has a "monopoly" on cocktail sales in its own building, or that I have a "monopoly" on lemonade sales on my front lawn.
If you have to apply an arbitrary constraint to eliminate the consideration of meaningful alternatives that the consumer has access to, then you're not dealing with an actual monopoly.