In technical terms, this situation is referred to as a monopsony [0]; the customer can, at any time, go out and purchase apps from other sources. However, app developers cannot choose to "sell" their apps to other retailers, since there are basically only 2.

It's a distinction without a difference in terms of the power they exert, but legally speaking, the distinction is relevant because the types of behaviors that are proscribed (in the U.S., at least) are relatively narrowly defined. This is not the first time these laws would have to be modified to account for shifts in market exploitation practices, nor will it be last.

For a recent discussion of the topic, see [1].

[0]: https://en.wikipedia.org/wiki/Monopsony [1]: https://www.nytimes.com/2021/04/24/opinion/joan-robinson-eco...