It just concerns some of the artwork, which is a major endeavour to recreate.
For example
https://github.com/ZeroK-RTS/Zero-K-Artwork/blob/master/musi...
is under a Noncommercial License, so you can probably fork ZeroK just fine, as long as you don't have commercial plans.
Unfortunately non-open source artwork is a problem with many games whose sources were released by the original publishers, after they abandoned any commercial plans.
For example I'm a big fan of OpenRA, but its artwork is still non-free.
I think ZeroK is a great game, and BAR probably also, so I didn't mean to distract from the great accomplishments of its FLOSS gameengine authors.
Unfortunately non-open source artwork is a problem with many games whose sources were released by the original publishers, after they abandoned any commercial plans.
For example I'm a big fan of OpenRA, but its artwork is still non-free.
I think ZeroK is a great game, and BAR probably also, so I didn't mean to distract from the great accomplishments of its FLOSS gameengine authors.