Patreon likely gets some users who first discover Patreon via the iOS app and who would never go to the Patreon website but who would be comfortable making an iOS in-app purchase.
And Patreon also likely gets some users who first discover Patreon via the iOS app and would be comfortable going to the Patreon website to subscribe if the iOS app didn't have in-app purchases.
I was referring to that first group, which I suspect is a small portion of Patreon's total userbase, but I suspect it's a niche that Patreon is very interested in (both because Patreon can't engage them through any other channel, and because they probably have above-average discretionary spending).
The whole "protecting users from themselves" thing is just a very tired argument. You might as well say that an operating system implementing memory protection is "protecting users from themselves." Or a software vendor offering security updates is "protecting users from themselves."