The 12v battery in a hybrid doesn’t start the car like you’d expect – it provides power to the computer, relays, ECU, etc. The heavy lifting of starting the engine is actually done by the hybrid battery.
I knew about the rest of the things it does because I can run my stereo and my headlights on my Prius without the battery indicator dropping. But I totally thought it also started the car.
Well in a technical sense you are also right, since the contactor for the main battery gets power from the 12v (albeit it's not a big load)
Meaning if the 12v is completely dead the hv battery will not switch on and the motor will not start.