In fact, they never say it’s not Linux. The summary would be more like: “ChromeOS should be counted among desktop Linux, because it has the same basis as any other GNU/Linux distribution, unlike Android. Here are the dumb reasons I think people would argue against this.”
Such definitions are becoming more and more complicated. I think we should standardize a name for the family of systems we use, or it will become uglier and uglier.
Recently, I had to write an academic work in the area, and an entire section was dedicated to explaining this controversy and defining what kind of system I was talking about, so that the work is reproducible.