Linux owns the web server market share so requiring Linux knowledge is pretty standard stuff for that job. I unfortunately can't read into that as anything more than assuring the software can communicate with a Unix/Linux back end for Multiplayer and updates.
Right, it can as well mean their server backend. By the way, don't forget to vote on the actual request linked in the first post if you didn't yet.
About Witcher 3 in 2015. I'm sure that it won't have a Linux version at launch, because CD Projekt Red didn't even start porting their Red Engine to OpenGL yet or if they did - they won't make it in time for the release. But there is hope that it will happen later at some point. So let's gather more votes to demonstrate demand.