It sounds like how ChromeOS is Gentoo based but does not ship the package manager.

It does I believe? I've never tried it myself but I've heard multiple voices say that once you go into the terminal the entire Gentoo stack is just there with portage, equery, qapps and such.

In fact, from what I understand it is in fact not really Gentoo based but Portage-based, as in they for the most part write their own ebuilds and software and from what I know have their own custom init system and display system that's not in Gentoo but they found that Portage was simply very convenient for automating their entire process. The claim that “gentoo is just Portage” is not entirely true, there's still a supported base system that's configured as offered by Gentoo but it's far more flexible than that of most systems of course, granting the user choice over all sorts of fundamental system components.

Your telling me google uses Gentoo for ChromeOS but doesn't even host a Gentoo mirror? jeez...