Huh, I have GrapheneOS and I never noticed it rebooting. (And when i manually reboot, the "BIOS" prevents it from booting without acknowledging that I'm aware it's a non-Google OS, so how does it work?)
The feature is not enabled by default. Also, the boot doesn't wait for you indefinitely - it just gives you a few seconds to glance the checksum and halt it, before it proceeds automatically.
You don't have to acknowledge anything. The boot screen shows a warning which you can interrupt. If you don't do anything it'll continue to load as normal.