I agree that if someone discovered the artist elsewhere, Apple has weaker standing in claiming a huge commission. But if they found an artist elsewhere, they would also know that they can support that artist elsewhere and not through the iOS app. If the patron found them through the patreon iOS app and use the app to consume the artist's content, then clearly the patron has indicated that they prefer the iOS experience.
I agree that if someone discovered the artist elsewhere, Apple has weaker standing in claiming a huge commission. But if they found an artist elsewhere, they would also know that they can support that artist elsewhere and not through the iOS app. If the patron found them through the patreon iOS app and use the app to consume the artist's content, then clearly the patron has indicated that they prefer the iOS experience.