I realized they might have added a fake reboot menu. So either use the exploit yourself to check it's the real bootloader (no realistic chance the FBI made a fake bootloader exploit in the fake reboot menu) or let the battery run out or remove it.

seems like a huge amount of effort when they could simply give you a bugged phone of the same model that automatically transmits the passcode to them when you enter it. Newest ios are usually vulnerable to Cellebrite anyways.