Richard Stallman has a policy of putting his non-engineering works under a non-libre license (CC-BY-ND 3.0) [0] [1], so maybe he wouldn't have a problem with this non-libre game as the underlying code is libre, just not the assets.

In my opinion, this is one of the problems with Stallman's "moral authority" approach to the FSF. I would very much like to hear about these cases where a game's source is libre licensed but not it's content and see what Stallman actually has to say about it.

Not for nothing, but there are many network/online games that have both source and content libre licensed. Here are a few:

* Teeworlds - https://www.teeworlds.com/

* Minetest - https://www.minetest.net/

* Battle for Wesnoth - https://github.com/wesnoth/wesnoth

I'm sure there are many others of note but those are just the few that I have at least some passing familiarity with.

[0] https://stallman.org/#thanks

[1] https://creativecommons.org/licenses/by-nd/3.0/