The low tile count simply comes from applying the dual grid approach popularized (if not invented) by Oskar Stålberg to a specific 2d perspective where tiles can be generated through rotations and/or flips.

Oskar does both rotations and flips to his 3D tiles, albeit only in 2 dimensions. He uses basically the same set of 5 tile shapes for any tiles that don't need to take into account verticality (i.e. look at the beach tiles in Bad North)