It is just an addon to X as I understand it?
No, it's a commercial light-weight fb-based gui -- it is said to do direct hardware access and be faster than X (with 2d applications I suppose) -- that recently got support for X applications. I.e. it is an X server now (or alternatively may contain an Xlib, that is just a wrapper for the direct calls, I did not dig deep enough). The source is said to be 90% free, but that does not make it free-as-beer nor free-as-speech, and I did not look long enough to learn, if they e.g. only mean the linux kernel and applications; after all, they sell a complete LFS based distro.
You could also run X on FB :)