Not knowing anything about how the project works, they state their read latency is on the order of 5us and write latency is on the order of 15us. On this synopsis, they also mention that one mechanism they deal with is scale. So, I think your guess correct in that they likely replicate writes across multiple nodes. Then if any single node goes down, they still have a valid valid for a given key. Writes could then be persisted to disk asynchronously.