Alright! I expected you to know that, but I thought it still deserved clarification for the record
I still think it deserves discussion, though. I don't know much about video constituting derivative works, but if it is so, a video must be GPL-licensed as well and the material bundled with it too. That means the artist is not only giving you the right to exploit the music commercially in your video (as per the GPL), but to anyone else downloading the source material to the video, which you must provide.
That reaches much farther than individual permission, and it could make the musician think twice about letting you use the track in a derivative of a GPL work.
I think there have been discussions of adding an exception to Xonotic's license to make it clear that videos (or a precise type of videos, to avoid loopholes) and (some type of) screenshots are not considered as derivative, but that might be complicated.
Anyway, since many people already post screenshots and videos of copyrighted works without suffering from it, Xonotic's probably not going to be the game changer there. Plus, there's fair use and other exceptions which put this problem in a gray area.