GP is right, in a sense. Chromebooks (the Intel ones, anyway) typically ship with Coreboot, and Google developers regularly contribute to upstream Coreboot.[0]

While you're correct that it's a pretty closed platform out of the box, you ultimately have more control over the platform than a typical x86 PC, if you go through the hoops of flashing your own Coreboot payload. The fact that it's hardware with official Coreboot support is notable, IMO.

[0] https://github.com/coreboot/coreboot/search?o=desc&q=google....