For one, it explicitly calls itself a “subset”; a subset is not the whole set.
If we don’t want to go just off the pedantics of language though, then here’s the thing: it was forked a very long time ago, and both have diverged significantly, I think. It’s a bit like saying Blink (the rendering engine of Chromium) is WebKit; sure, Blink is a fork of WebKit, but the two are very different now.