So here's the thing. Chrome OS is absolutely a Linux distribution. It's even a GNU/Linux distribution (based on Gentoo), unlike say Android/Linux, which is doing its own thing. But... When people say "desktop Linux", they mean one of the user-controlled distros, not the locked down appliance that is ChromeOS. It's like how nobody who says they want a Linux phone mean Android. Technically correct, but not what people mean.