I don't think the license says the keys are part of the source code. It says they have to provide information on how to run your own modified version of the sof

I don't think the license says the keys are part of the source code. It says they have to provide information on how to run your own modified version of the software. In this case it would be as simple as allowing the user to install their own key or disable secure boot. If an OEM didn't do that and pre-installed Ubuntu on the machine there is no way that Ubuntu would be required to reveal their private key, they did nothing wrong, the OEM is the one who violated the license and they are on the hook for damages.