but Spanner isn't Postgres compatible, they have their own transaction processing layer that is built for TrueTime. If this is using the Postgres frontend how are they incorporating TrueTime into the normal Postgres MVCC model that uses the monotonic xid.

edit: More details make it seem like this _isn't_ Postgres on the frontend, its just mostly postgres compatible, which would explain how they got away from the xid based MVCC. So it seems like this is an entirely new distributed database, rather then a modification to existing postgres.