> How can the app store promote competition when it is a monopoly on iOS? (I suppose they could be talking between different apps, but that isn't what this case was about)

No need to go that far, the couple Google / Apple is a duopoly without any competition, regardless of their marketshare. So you can say that there's no competition on mobile marketplaces at all.

(I'm ignoring irrelevant marketplaces such as Samsung store or Amazon app store which are a rounding error at best)