ChromeOS, which has another 4%. Which is also Linux but the "wrong" kind. It was the author's tongue-in-cheek way of highlighting the fact that ChromeOS is so different for the regular Linux user that not being counted as Linux actually makes sense.
I read the title and assumed it was a typo, made the joke, then clicked into the article to see that no, that really was what the title was saying. lol
Deleted the comment immediately but apparently it is still there for others?