Again, you seem to imply a retrofitted coreboot. If you can tell me any model with a firmware lock in particular, I can try to compare it to the coreboot situation for that model.
I think the most common retrofitted coreboot solution that people use is for older thinkpads. So it seems reasonable to provide some additional guidance for people attempting that in the official documentation. The threat model in baseline is that even if the OS is compromised, it cannot write to the bios. The vendor's signing keys are considered trusted in this model, exploits notwithstanding. i.e., You can get to a clean slate by wiping your hard drive. So with that it mind, it may be useful to help people achieve a similar goal. In general, the landscape of different security measures such as vboot, heads etc. is hard to grasp for lay people. It is not immediately obvious how to use them, or specifically how they differ in threat models, whether they allow subsequent flashing without using an external programmer if the hardware doesn't have dedicated hardware like chromebooks, whether it is possible to have a clean trusted/airgapped machine for just building and signing coreboot builds which would be the only trusted builds by your target devices, etc.