Fourth paragraph of the article:
> Chrome OS is Linux under the hood so, in principle, you can run any standalone Linux binary on it. More precisely, Chrome OS is based on Gentoo Linux.
Correct, Chrome OS is a Linux-based distribution. I believe it is/was based on Gentoo although I may be misremembering that.
You are correct. Gentoo was selected for its Portage package management system.
They don't immediately explain that in their docs and, being a web dev, it's hard to tell from the outside.