The storage abstraction layer is also a shared concept, even down to providing only a free RAM implementation.

My first thought too.

But looks like this may be somewhat more oriented towards including temporal information in queries, which implies a slightly different indexing model to Datomic (where time is the last component of all the indexes the Datalog query system accesses).

I think using the term open source alone can be a little bit misleading: https://my.datomic.com/downloads/free