They probably are just trying to hide a hard coded signing key, but that's the whole point of the GPL isn't it? That you can't get away with that kind of crap. The GPL exists to keep the ecosystem open for the people that want to use it, and prevent big portions of it from being co-opted by commercial organizations and hidden behind DRM like signing keys burried in bootloaders.
If someone wants to hide their firmware behind a hard coded signing key they have to write they're own boot loader, they're not allowed to use a GPL'd bootloader and then just ignore the rules because it's cheaper to steal someone elses.
And I think your legal analysis is incorrect, the gentlemen who wrote this blog entry doesn't have standing to sue and wouldn't be part of the case. My understanding is only the original copyright holder has standing to sue.
The original copyright holder wrote a threatening letter a year ago and did nothing. If he did sue he would win but get nothing, and probably doesn't think it's worth the trouble. Ubiquity is banking on him thinking it won't be worth the trouble because they know if they get sued and loose all they have to do is what they should have done in the first place.
Back in the day the GPL used to have a nuclear option that said that if you were found in violation of the GPL by any court you lost your ability to distribute any GPL software from that point on without the explicit permission of all copyright holders. Back in the day, the GPL had some teeth, and corporate legal departments did't fuck around with it like this. That was considered too extreme, unfortunately, and new versions make being found in violation pretty harmless.