There is a sidebar about sharing subscriptions in the sync documentation.
Long story short-ish: Mac and iOS App Stores do not interoperate…so there are options to purchase the subscription on either platform, and if you have purchased it on one platform, it can only be restored via “Restore Purchases” on the platform you purchased it on…and, because of this, purchases cannot be shared simply by being on the same App Store account.
Drafts implements its own method of sharing subscriptions via iCloud. To share you must be using the same iCloud account (with iCloud Drive active) on both iOS and macOS. e.g., if you drafts data is syncing between accounts, your Pro subscription can be shared the same way.
If you are using the same iCloud account on both, you should not need both subs and can cancel the Mac one.
Does this help explain?