In practical terms, ChromeOS is radically different from every other Linux (desktop) distro. The usecase is very different, and the day to day handling of the system is also very different.
It makes sense to track ChromeOS in it's own category.
It's not so different than a modern immutable distro with Wayland and Flatpak apps, like Fedora Silverblue.
Sure, some of the daemons are different, like upstart instead of systemd. And out of the box it supports Android apps and doesn't allow shell access outside a container.
It's managed and owned by Google. That's what makes it different. I don't agree, but there's a clear us/them divide happening with Chrome OS right now that's also generational.
The vast, vast majority of ChromeOS users are children/students and teachers. ChromeOS exists so that Google has more avenues of data collection, so that they can sell schools on their ecosystem, and so that kids grow up on Google products (Drive, Docs, Sheets, etc) instead of Windows and Office. ChromeOS is a very legitimate threat to Windows/Office. Traditional desktop Linux isn't.
Howso? I use Brave. If I install Brave in their Linux system, it appears like a native app. It runs and reports as Linux. It doesn't even know it's on ChromeOS. I imagine Firefox is the same.
What i meant is that ChromeOS is primarily an OS for (young) students/kids. The primary usecase is in an education environment, and that is the angle from with which they compete against Windows and MacOS. Furthermore, ChromeOS is arguably much more of a threat to Windows/Office than traditional desktop Linux is. Hence why it makes sense to track it seperately.
It makes sense to track ChromeOS in it's own category.