You don’t really need two ground wires, CP has a diode with series resistance 2.74K to PE (ground) so it’s trivially possible for the EVSE to just send a push/pull waveform over CP and if it reads push but not pull (remember diode) then it knows it’s connected properly.

The return path for the pilot current will be over the PE contact, so it can also be detect and doubly verify that PE is low resistance, although ideally you’d want a much smaller series resistance.