Is this related to Xdebug?

Actually, no. Xdebug is another great debugging application and not related to this project. The similar name has nothing to do with the other application, no matter what the people say.

xn - stands for Xell-Network

It is the name of the origin software package, where xn.debug was born from in 2005. The focus of Xdebug requires a server library to run and allows a very good IDE integration. xn.debug instead is build to work directly without server libraries and focuses on the on-demand debugging. Both tools have their advantages and could be used parallel if needed.