I haven't used Datomic, but you're right that the part that requires over there is "single consistent source of truth." There's only ever a single node that is sequencing all writes. Perhaps as a result of this, it provides strong [verified ACID guarantees][1].

What I got from Hickey's talk is that he wanted to design a system that resisted the urge to encode everything in a stored procedure and run it on the database server.

[1]: https://jepsen.io/analyses/datomic-pro-1.0.7075