I am not sure what you're trying to tell me with an RFC pertaining only to CGNAT. We're talking about transit providers running core and backbone routers here. Such routers aren't running NAT of any kind, and they aren't customer-facing appliances.

The transit provider needs to own some IP space to assign addresses to router interfaces. This is how internetwork routing works. It goes from one network to another, and the router must be a member of the source and destination network in order to participate in each hop.

A transit provider doesn't need many IP addresses in total; if all they're running is core routers then they need sparse allocations: a point-to-point link only needs a /30 or /31 to function correctly. They'll still need to own a variety of netblocks, though, because if they offer transit and backbone services then these will be geographically disparate, and they will necessarily transit a large amount of logical address space.