Reading through the site reminds me of append-only CouchDB (or even better, BigCouch) both Datomic datoms and CouchDB documents have time stamps so the state of data is available for different times.

This looks new: local query peers that cache enough data to perform queries (I don't understand how that works, but it looks like indices might be local, with some data also cached locally).

Also interesting that it seems to use DynamoDB under the hood.