No, thats not true. X is a GUI, but its not the only one, it can use the framebuffer/VESA/hardware just like anyother GUI, and provide other features, like windows to use.
Most other appempts at a full GUI are not so full, as their just isn't developers for them to really complete the other GUI's to a point where it can be used like X (such as getting a DE setup for most modern users). On the other side, you can use the framebuffer (a console) and run some programs that know how to use it, some text mode browsers can use the framebuffer, for example, as well as some image viewers, and some video players (mplayer). Then you have projects like directfb, while not really being a full GUI environment, they do offer some interesting GUI options for the framebuffer (but no toolkit, altho gtk has been ported to it, and i hear some talk about fltk also being ported to use directfb (but thats likely years away), mplayer also works under directfb (but not much else, unless you like gtk applications).
i guess it all depends on what you need/want.
http://www.linuxdevices.com/links/LK4761626139.html
http://en.wikipedia.org/wiki/Graphic...onment_Manager
http://en.wikipedia.org/wiki/Y_Window_System
altho when i looked into other window systems a few months back, the list was bigger, but i cant seem 2 remember what to search for that got me all the good results. (some of which look very interesting, but doubtful if they will ever replace X, only because everything works for X now, and changing all the applications would take a long time)