What if it isn't an app, though. They're still taking a cut of it, even though they're not paying for anything related to apple. The only thing apple is doing is hosting the patreon app, not the thing being bought.
And if you think that it should be different when buying something that runs on an apple device, logically that should also apply when buying it from a computer.
Guitarist sells $10/mo subscriptions to their guitar tabs through the Patreon app.
Why should these be considered any different?