This seems to be possibly an earlier attempt at creating functionality that is now covered by VisualEditor ? There is some functionality covered by this extension not covered by the VE but VE seems to be getting more mainstream support and is more current? A part of this page clarifying this information would be helpful to new devs or sysadmins looking to get more involved in working with the MW ecosystem.
What's the difference or relationship of this extension to the Visual Editor?
I am not familar with setup or properties of VisualEditor so I can not compare it to Wysiwyg. As far as I can see, Wysiwyg is easier to setup. It has its limitations but works quite well with "standard" wiki pages.
There have been only couble of contributors working with this extensi lately.n.