How would that be helpful if you want to cancel the subscription using a third party payment processor from the app store/settings(current workflow)? Apple would still have to call a third party API, otherwise it doesn’t seem particularly useful (i.e. you just get to see the status and that’s it?)

That only works if the external API is handing off the entire subscription to Apple, up to and including payments. But the entire premise is to move away from being forced to use Apple for these elements, which makes it a non-sensical interpretation. In fact, that particular interpretation is the current status-quo -- apps use APIs to create subscriptions entirely managed by Apple.

If Apple does not control the actual subscription, but is only providing an interface for managing it, then Apple must then alert the actual owner of the subscription upon changes. There's then no guarantee that the code on the other side is properly handling that alert.