The external model is much simpler to do for Redis than a database with complex cross-key queries like an SQL database. Redis has scans but no queries or “real” transactions with rollback. To me it sounds like more work to implement MemoryDB and then additionally re-implement the Redis interior than to implement MemoryDB and use the off-the-shelf Redis.

I also think that the decoupled design is kind of elegant, it allows the logical implementation to be developed independently of the durability bits. It’s open-core but someone else is building the core.