I encourage you to read the original article.
Couple things:
1. The argument isn't that it's a monopoly, it's that it's arguably monopsony. The issue is less that users don't have the choice to buy the phone they want with the OS they want, it's that developers/suppliers are unable to access a huge chunk of the market without being forced to pay 30% of earnings.
2. While it's closer to a "duopsony" than a "monopsony", it gets a little more complicated when you look at the level of each regional market. In the US, Apple's market share is closer to 50%.
Maybe it's not anti-competitive for there to be only 2 app buyers on the market, but maybe it is. That's the debate, and it's by no means clear cut.
