Comment by mrpdaemon

CoreOS is built using Gentoo. Gentoo is a meta-distribution whose main contribution is the portage tree which is a collective of ebuilds that describe how to build each individual package, the dependencies, the configuration flags etc. Literally every user who uses Gentoo builds and produces their own linux distribution, with their own CFLAGS, USE flags etc. ChromeOS is built using Gentoo, so is CoreOS. Neither ChromeOS nor CoreOS are Gentoo based distros because Gentoo is a toolkit to build distributions, not a distribution itself.