Right the term functional is just confusing. Temporal database theory is relatively mature and even part of the SQL standard for which IBM implements in DB2.

Any database that uses a append only log can potentially be temporal so long as it retains the log and exposes the information in the log (which most don't).

Even say PostgreSQL uses a form short term temporal querying in order to provide MVCC, to bad they haven't taken it further to allow fully temporal standards.