http://www.xaraxtreme.org/Developers...tributing.html
"The Xara LX source code is released under the GNU General Public License (GPL)."

The wikipedia page describes multiple products. Some of them, Like Xtreme are proprietary, while Xtreme LX is comletely open source.
In other words, entire source code of the Xtreme LX is open and released under GNU. It does not use any proprietary code.
The Windows Xtreme is a different product with different licensing.
But I though you were talking about LX, don't you?