apple pay expressly forbids being used for digital content delivered within apps.

Right, but the only reason for that is it prevents circumvention. I would assume after this injunction they will allow using apple pay in order to re-capture some of that revenue.

If they want to keep this rule, they can- but they're just losing out.

Will I be able to use it with King's implementation, or my local pizza place's outsourced app?

edit: I was instantly downvoted without any discussion, which doesn't add to the conversation. The reason I ask is because that's what the key value add is of Apple's IAP - I know that my payment method is accepted on the app store.