It's half true, there are plenty of android auto apps on the store. The majority of them are music playing apps. Navigation apps are not allowed by google. I have 2 cars with android auto and android auto is useless for me because i cannot mirror the phone screen.
Apparently that used to be true but is no longer. OsmAnd (OpenStreetMap navigation app) is releasing their Android Auto app later this year: https://github.com/osmandapp/OsmAnd/issues/3391
