mods clause is in the EULA that comes with the game when you buy it, iirc. the game assets can be modified or even replace for a mods but that doesn't make a entirely new asset, meaning that you still require parts of the original asset to be distribute, making a standalone using modified asset(s) is strictly forbidden. EULA remains valid for the game and its assets while the source code of the game use GPLv2 license. Hope that makes sense.