#2 and #3 are fearmongering arguments and total horseshit, excuse the strong language.

Should either of those things happen the bootloader puts up a big bright flashing yellow warning screen saying "Someone hacked your device!"

I use a Pixel device and run GrapheneOS, the bootloader always pauses for ~5 seconds to warn me that the OS is not official.

Yes. They're making the point that your flashing yellow warning is a good thing, and that it's helpful to the customer that a mechanism is in place to prevent it from being disabled by an attacker.