Functional programming is very closely tied to the notion of immutable data. "Don't modify data in place" is one of the pillars of the functional model.

> It seems that Hickey just reinvented time series and tries to sell those as database.

Datomic is much more than a time-series, although that's the aspect that arguably makes it "functional." For example, it provides a datalog query interface and has a pretty fascinating and unique architecture that provides extremely high scalability for queries. If you don't like Datomic, or if it doesn't suit your purposes, or if you think it's overpriced for its value-added (I do), that's cool, but don't dismiss it based on a malformed understanding of what it is.