First, he/she didn't install a different OS. He removed a Linux distribution (yes, that's all Chrome OS is, and Android for that matter) and installed another Linux distribution. Chrome OS and Android are nothing more (stress, NOTHING MORE) than a stock Linux kernel stripped down severely and very little outside maintenance of device drivers from Google. And the only reason they are maintained outside the mainline Linux kernel is because they we're removed from it for "bad code and bad maintenance".
Secondly, as you probably all already know, Ubuntu is yet another Linux distribution. However, it is about as representative of the GNU/Linux community as OSX is as representative of the BSD world. It took the Apple route and made it moron proof, but as history shows us, that tends to be pretty popular. Coincidence?