They restrict use to a signed OS through their payload. coreboot only does hardware initialization, then passes control to other software in flash that handles

They restrict use to a signed OS through their payload. coreboot only does hardware initialization, then passes control to other software in flash that handles loading the OS and policy surrounding it. As for the "Supported Motherboards" list, it's automatically generated from reports that are sent in through a tool. Google doesn't do that (yet?), so they're not listed as supported. Still better than our old, manually maintained list, that got stale fast (with false positives, which are much more annoying from a support standpoint than false negatives)