I honestly don't think this is true. I think people tend to discover a creator they like first, and then click through to their Patreon. I don't think the reverse happens very often; I don't think many people install the Patreon app so that they can look through lists of people they could give money to. And if they do, they're clearly aware enough of Patreon to be able to use the website instead.

Assuming the average user accesses Pateron by clicking a link, there's really no reason for a dedicated Patreon app to exist. Apple Pay works just fine on websites, without demanding the App Store fee.