I know what open source means.

A group that wants to host that game server is eventually going to need to fix it, or adapt it to new software / hardware if we expect it to be able to run in perpetuity. They will need to be able to modify that source code to do so.

So the game publisher can’t just publish one final compiled package “for people to use.” The software has to be able to be maintained, not just used. And if we expect the software to survive over time, it will need a community not just one group. Open source is the only thing that would allow the game publisher to walk away from it, but allow the software to live on.