It's not just a new X WM, though we do still use X (at the screen level). Basically, we render everything inside the X root window. It's all composited, using the same compositor we use for web content ("CC").
ChromeOS used to have an X window manager (which is what your link is pointing to, the code used in M18 and earlier). The cool thing about the new code is that it runs in Chrome, so it's much easier for more engineers to test and develop for, and it's also platform agnostic (since the only X interface is at the host layer). This means Windows developers like myself can work on ChromeOS WM features since I can effectively build and run the ChromeOS WM on Windows.
