Schools are a HUGE user of Chromebooks. Most highschools in the US lend each of their students a Chromebook to use, and that alone accounts for a huge userbase.
Sure, in the same way Android is a subset of Linux. Their appeal and use-cases are completely different, and in no way are GNU/Linux and ChromeOS practically interchangeable.
That's like saying ketchup is a fruit because it's made with tomatoes.