I don't have any direct experience of building a business on Android. Last I heard iOS users spend more money on the App Store than Android users. I think this implies it is harder. I would think they are both pretty difficult probably in the same order of magnitude to each other.
So: on net, the single distribution channel provides access to paying customers, in turn making it easier—-not harder—-for indies to build a business there.
Which makes it wrong to say iOS’s single distribution channel is bad for indies: it’s actually good for indies.
Unless you claim that walled garden and paying users occurring together is a sheer coincidence.