Yeah I strongly disagree with the idea that game art is "optional" or just a "nice to have". This doesn't make sense to me. Without game art, you have no playable game. The game engine might not even start and just crash instantly if the game art files are missing so the engine is not useful even in theory.
Think of Xonotic but without maps, player models, textures, music, sound effects, texts translations, etc. Is that still a game? Can you play it? No.
Now think of Xonotic again, but this time, all maps are CC BY-ND, i.e. under a non-free license. Is that still a game? Yes. But is it still a free game? I'd argue, no. For a software to be free, all components in it must be free. In source code, this is very strict. A single proprietary line of code is already too much.
My argument is, non-code files, like level files, maps, etc. are a de-facto component of the software, and it's an essential component, too. Remove all non-free files (in this case: the maps) and the primary purpose of the software (gaming) is no longer practically possible. Xonotic ceases to be a free game the moment you taint a single line of code with proprietary code. Likewise, it would cease to be a free game if you taint it with non-free data.
So if "libre" just means a "stronger version of free software", free software that includes artwork, then that definition makes no sense to me tbh. Because to me, free software implies free artwork if it is a component in the software. Especially in games, artwork is critically important that is absolutely neccesary to make the game work, it is not just an "additional contribution".
Btw, LibreGameWiki also treats the word "libre" as synonym for "free".
The term libre is sometimes used as a synonym to free to prevent this confusion.
https://libregamewiki.org/Free_games
Also:
"Libre" is a subset of "FOSS" with stricter requirements as seen by the exclusion of several FOSS games from the LibreGameWiki.
This claim is just flat out wrong. I have not seen any FOSS game that has been excluded from LGW. Every exclusion was justified because the game was found to be in conflict with the free software definition, usually there was a missing or non-free license (rejected by FSF and almost always OSI, too). They don't make a distinction between "libre software" and "free software". It's the same.
OK, when I think about it, I feel strengthened in my belief that "libre" is just a synonym for "free software". But I'm still open to other definitions, if they seem logical. Just in case I ignored some other aspect.
Now whether it is good or bad to ban games with free source code, but non-free data, that's a completely different discussion, of course. I'm just arguing about terms here. :P