ChromeOS is an immutable Gentoo installation. The offered additional Linux environment is, understandably, based on Debian in order to allow for the most compatibility with .deb packages. Of course you can't natively install .deb packages on Gentoo and especially so if you don't have the rights to install spurious packages on the system, so Google "has to" offer the Linux experience in a roundabout way.
Of course, they could base ChromeOS on Debian directly and be done with it, but then they'd lose the incredible ease of custom tailoring to the hardware that Gentoo offers.
Of course, they could base ChromeOS on Debian directly and be done with it, but then they'd lose the incredible ease of custom tailoring to the hardware that Gentoo offers.