tl;dr: the art/sound/etc. assets may have been accidentally released under the GPL because of a file omission in the .zip version of the source release.

From the article: "According to Kotaku, they are talking about the GPL release of the source code for a number of the Humble Indie Bundle games, namely this post. However, the license made it very clear that the authors retained all rights to the assets, characters, and everything else aside from the code itself."

Disclaimer: I'm not a lawyer, I might be wrong here about some things, so please step in and correct me if anyone knows better.

So, here's the thing. Looking at the source .zip linked at http://blog.wolfire.com/2010/05/Lugaru-goes-open-source (the .zip file itself is at http://akamai.wolfire.com/humble/src/lugaru-srcs-final.zip, so that's an official release), I see COPYING.txt, the generic GPLv2 statement, sitting at the root of the project .zip. By custom, I would naturally assume that unless that license is overridden somewhere, it's the license in effect for all copyrightable materials included in nested directories.

I'm not positive about this legally speaking, but I'm pretty sure that if the code is distributed in such a fashion, and most people familiar with code would interpret the license to apply in such a manner (to everything in the subdirectories, unless overridden somewhere within the released code), I can safely assume I've been granted that license.

Now, I fully believe that Wolfire never intended to release the art/sound/character assets under the GPL. But - and I encourage people to check for themselves to make sure I'm not missing something, I could definitely be wrong - there appears to be no license statement anywhere inside that particular source distribution (http://akamai.wolfire.com/humble/src/lugaru-srcs-final.zip) that puts any of those assets under another license.

I suspect the file at http://hg.icculus.org/icculus/lugaru/file/97b303e79826/CONTE..., the one that sets the license terms on that data, is what the author of the post was referring to when mentioning the clear license terms.

But someone forgot to copy it from the hg repo into the .zip file before they released it.

Unfortunately, they didn't forget to put COPYING.txt in at the root, which means GPL2 for the whole shebang (the default license would have been none at all, but for the presence of that file). If I'm right, and no other license is hiding somewhere, then as far as I can tell (not a lawyer) those assets are under GPL. It doesn't matter if comments elsewhere clearly prove that the GPL was only supposed to cover the code, a release was made and linked by the original copyright holder with a license statement whose clearest reading grants the user a license to everything under the GPL. Prevailing wisdom seems to be that you can't take back a GPL license, so yes, that means that all the assets in this game have been accidentally open sourced.

There's still a trademark claim, of course, this joker can't legally call his game Lugaru, but if he renames it, he's within his rights to try to sell it as long as he branched from the .zip release (though the GPL issue might keep it off the App Store).