Well, I’d say libre software comes with an activist approach on this kind of non-sense. It would be appropriate to make some kind of statement regarding this issue.
Why not releasing assets? Or looking for free/open assets in the first place?
Why the code would be ‘just for reference’, if the game needs to be ported or if you lose the source code, releasing it under a libre licence will be pretty useful.
Ok, regarding to the assets you can release them under their initial licence with the code on another one. It’s not a problem.
You can even publish things under multiple licences if they aren’t exclusive (I’ve done this on my game).