Sure they can use proprietary technology in their backend. They just need permission to redistribute it, like is currently the case with any proprietary technology used by the client.

Alternatively they can use proprietary technology in the backend without permission to redistribute, so long as it is replaced before support is ended.

They are allowed to use that. There just needs to be a way for that proprietary technology to be bought and used on a backend by a user if the company deems providing the backend to be costly to continue. That's my understanding