So... really no. The overall spirit of the GPLv3 text is that the license demands preservation of the ability of an end user to modify and run the software they receive, see all the discussion at the time it was being drafted about "TiVoization". It does not specifically speak to encryption and signatures being used by the loader environment, but it's absolutely reasonable to interpret it that way.

If I buy a laptop from Lenovo, and it uses a signed GPLv3 bootloader, and I want to modify and run a new version of that bootloader, I'm prevented by the lack of a signature. That action is exactly what the license demands Lenovo permit.

Now, obviously this collides badly with the security requirements of the system. So it's likewise totally reasonable to imagine a court flipping the other way and refusing to literally enforce the GPLv3 because of the potential damage to the market. But if that happens, then what gives Lenovo any right to ship that GPLv3 software at all? Now they're subject to the bootloader authors demanding they stop shipment of the infringing product.

No, the permissively-licensed shim compromise was the right choice.