Its not pressure to release the source code. Its that they need to release the server so anyone can run it.

From Day 1 any Doom client could be a multiplayer server and this is how it worked for almost all games - Descent, Quake, C&C etc...

Right, but presumably the Doom and Quake server code was written by id Technologies themselves. That's not the case with a lot of modern multiplayer games. They license middleware like Photon Engine and don't have the rights to redistribute the server software, even in binary format.

I guess they could just strip our the parts of the server code that they don't have the rights to redistribute, but then it wouldn't be functional.