If Ubuntu doesn't distribute GPL code to end users then they don't have to provide source, keys, etc to them. Whoever does distributed GPL code to end users does have to do this, and anybody with a copyright to the code can sue them if they fail to do so.
Now, if one of the hardware vendors were to ask Ubuntu for their signing key, then they'd have to provide it to them. So, this isn't going to be of much help...
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