No, I think you misunderstand. It's not "can you use it elsewhere?", it's "if the user chose to, could they use the purchase solely inside the context of the Apple ecosystem?"
That is, if someone wants to, they can use Patreon's iOS app, not interact with Patreon in any other way, and get all of the benefits available to patrons.
That is, if someone wants to, they can use Patreon's iOS app, not interact with Patreon in any other way, and get all of the benefits available to patrons.