How so? That link describes a Monopsony as a situation where there is only one buyer; are all iOS device users "buyers" from the App Store?
Developers are the sellers
Apps are the goods
App Store/Apple is the buyer
Yes, Apple is both a monopoly and a monopsony for Apple devices. Just like Marks & Spencer is for Marks & Spencer products. But M&S isn't the only supermarket and Apple isn't the only smartphone manufacturer. In fact both have pretty low marketshare.