I too feel a little weird about "iOS app ecosystem" as the definition of a market that matters for anti-trust regulation, but at the same time consider that smartphones are not fungible goods. People generally choose iPhone vs. Android for particular reasons, and then often stay on those platforms because of features that aren't available on the competing platform, or just because they've succumbed to ecosystem lock-in. Switching to the other platform isn't just a matter of swapping out a piece of hardware: it involves installing or finding workalike applications, often manually re-entering data, and sometimes losing access to DRMed purchases.

Given that, "iOS App Store" does start feeling like a market that should be regulated as its own entity.