I tend to see Tarantool as a Lua powered database. In this sense, you could easily implement a Redis like system on top of it using Lua

I don't think so.

Tarantool uses an async evented IO model, but uses Lua coroutines and not Javascript. There are not callbacks, just 'yield points'.

Also, the primary data store backend is an in-memory database with optional 'snapshoting' to disk. An alternative backend uses sophia, so it's not 100% in memory.

The latest. Sophia was created as a disk-based engine for Tarantool, and is also available as a standalone embeddable library