WalteriusMaximus This statement by
ImprovizoR is simply not true.
An excerpt from the wcc_eula - the license agreement between CD Projekt RED and the mod maker who installed the mod tools:
So a mod is copyrighted by the mod author, period.
And any license, permissions etc. a mod author decides to put onto his mod is an agreement between the author and the mod user, which the user has to adhere to. So redistributing a mod without the author's is actually an act of (the exact legal definition may vary depending on the country you live in) software piracy or theft of intellectual property.
Even in regards to modding Bethesda games the statement is wrong. Custom assets have always been copyrighted by their creator, in addition to that the new Creation Kit EULA (since Fallout 4) includes a similar paragraph to the quote above which now clarifies that even content created without custom assets is copyrighted to the mod author and only sub-licensed to ZeniMax.