I would put it the other way around: InnoDB is a backend for MySQL. But an SQL database is far more complicated than a key-value store, and has a lot of "middle end" between the "talk to the client" front end and the "store stuff on disk" back end.

I'm not saying that RedixDB isn't interesting -- having a network front-end to Badger seems like it would extremely useful for most people who want to use Badger. It's just that I spent a while digging through the code trying to figure out where the back end was and what data structures you were using, only to discover that you didn't have a backend (or rather, you're using a pre-existing one).