As your question is vague, I broke it down into 2 questions for you:
a) “If Graphene is flashed, how can a backdoor in the Google hardware exist?”  
The answer is that all hardware uses firmware which operates at a lower, more base level, than an operating system.  This firmware could potentially communicate to remote actors.  This could potentially be EVALUATED through WiFi to a router you control, but can not be even evaluated if it goes to cellphone towers.  It also could potentially communicate even right under your nose of a FOSS router firewall, if you go to “google.com” and some hidden data exchange takes place with SSL encryption to the right domain.

b) “If google’s push notifications are enabled, how can this google service get to the hardware identifiers on the device?”  The answer is that the sandboxed google push service prevents it from getting to the hardware identifiers IN THEORY.