You have to buy a Chromebook to run ChromeOS, right? You can install Linux on your PC as dual boot. The investment is far smaller. However you have to buy a Mac to run MacOS so maybe that means that a Chromebook is not a very compelling machine to buy, or has a limited use case.
I don't believe this for a moment. I have seen many more Chromebook users than desktop Linux users. Not individuals, but a lot of schools use Chromebooks where I am. They are probably not well-tallied in the results, because they are often used to do some work in Google Docs/Sheets, plus some education-specific sites and less browsing on the general web.
I guess my point is that it is nearly impossible to get reliable counts from user agent statistics, etc. (lots of crawlers, that identify as a regular desktop as well etc.). The best way would probably to do real-world surveys among random population samples. But that costs money/time.
If you increase the date range to 12 months you can see that ChromeOS is several times larger than Linux but recently there must have been an update so ChromeOS is now incorrectly detected as Linux.