In fact, Apple is already pretty clear in its developer agreement [0] that the 30% commission is for "its services as Your agent and/or commissionaire" (Schedule 2 3.4), not for its services as a payment processor. They are contractually allowed to take the 30% fee out of payments collected, but merely using a different payment processor doesn't remove the obligation to pay them for their other "services as Your agent and/or commissionaire".

[0] https://developer.apple.com/support/terms/apple-developer-pr...