"but in the event
that a manufacturer makes a mistake and delivers a locked-down system
with a GRUB 2 image signed by the Ubuntu key, we have not been able to
find legal guidance that we wouldn't then be required by the terms of
the GPLv3 to disclose our private key in order that users can install a
modified boot loader. At that point our certificates would of course be
revoked and everyone would end up worse off."
IANAL, but I don't think that can happen. It seems to me that in the above scenario, the manufacturer would be in violation of the GPLv3. Assuming a relevant copyright holder stepped forward to enforce the terms, the manufacturer would have to cease distribution and could be forced to pay some sort of damages. But I doubt a court would be inclined or able to force Ubuntu to do anything, if Ubuntu wasn't the violator.
It sounds like a spin on the old "if I accidentally link my code to GPL code and distribute it, then the GPL will spread to my code", which of course is not the case.