What's probably happening here is that you are paying Apple using Apple Pay. Apple is the merchant of record for in-app-purchases.
So it's not entirely incorrect (but definitely confusing) to say that the end user is "paying using Apple Pay", but they're paying Apple, not the app vendor.
So it's not entirely incorrect (but definitely confusing) to say that the end user is "paying using Apple Pay", but they're paying Apple, not the app vendor.