Nope, I mean, yes, you can talk with me, but I’m not an actual member of the Exiles development team
From what I read, @eNTi clearly stated from the start that he was talking about the Game Server itself, not the Launcher Tool.
I do believe there was a Linux version of the server internally at some point, but that there was some issues with it.
Regarding why it’s closed source, well, it’s not just the game server, the game server is a special build of the actual game, so releasing the game server source code would be like releasing the entire game source, including a bazillion of things that Funcom would not be legally allowed to release anyway.
Regarding the performance issues, they are not Windows or Linux related, there are people looking at it, that has more to do with the fact Exiles was built on a now quite old version of the Unreal Engine, which did not benefit from all the optimizations introduced by Epic later, so even if you had the source code, I doubt you could find magical optimizations that make the game run much faster
That being said, if you are an experienced coder with a track record, I guess you could still contact the team with some suggestions, eventually even sign some NDA to see if you can put your hands in the engine - does not hurt trying, at worse nothing will happen, at best it will happen and you will find some good stuff in there