It's not a simple sharing. It's actually share-nothing architecture and doing multi-key operations atomically is pretty complicated thing. I used paper from 2014 to solve this problem

Maybe simple is the wrong word (perhaps I should have said familiar) but Redis sharding is similarly shared-nothing, is it not?

In Redis cluster the client needs to be connected to all shards and manage those connections. Multi-key operation on different slots are not supported etc... Maintaining a cluster is not a fun responsibility. DF saves you in most cases from the need to grow horizontly which should be much simpler to maintain and work with.

So is it architecturally different or does DF just have a much better UX?

it is architecturally different. DF has the orchestrator underneath. The general. The guy that handles all the incoming requests and makes sure they execute atomically. Redis cluster is an orchestra without the orchestrator. Everyone to himself. It's share-nothing that is exposed outside. To actually hide the complexity in a *transparent way* is a difficult problem. Researches write articles on how to solve this problem efficiently.