Isn't the problem that the code would work fine on a piece of hardware that doesn't perform the signature check, it's just you'd have to make that piece of hardware yourself?
This is apparently not how the license had historically been interpreted by everyone, at least before GPLv3 appeared.
The intended and historical interpretation seems to have been that as long as you're distributing a device running GPL software, you have to provide the source code for that GPL software along with working instructions about how to build, install and run those sources on the device.
This even goes back to the original motivation for creating the GPL in the first place: the desire to repair faulty software in a printer at MIT.
I have no idea if this provision has ever been tested in court.
