There's technically no way to "swap" the provider; you can't extract payment details from the Apple-provided subscription infrastructure (partly because there might not even be payment details - they might be using carrier billing or iTunes gift cards).

However I don't see anything that suggests that's what they're trying to do - what I assume they're after (which would be very reasonable) is to offer direct card payments in addition to Apple's option (with the direct option priced cheaper to offset Apple's cut).

That's not what the ruling stipulated-it just says the developer must be given the option of presenting alternate payment providers. Presumably the court thinks the developers thus should be able to push an update that includes this additional functionality, whereas Apple's bizarre efforts at [non-]compliance require apps with non-Apple payment option to be a separate SKU (which is not really "develop a separate app" as they've described it here, but certainly is a barrier Apple has chosen to erect in an attempt to undermine this ruling.)

Apple already requires that you offer their login service if you offer any other ones, so I could see them imposing that requirement. But then you'd have to be able to compete with their option, like by offering lower prices, and they don't allow that.