Hi! Under assets that can be used in commercial projects (even free) there is the text you mentioned above. However, if your game/show/video/presentation is not a paid one, you can leave prototyping sprites in public available version. Third way is donation, that is exception from this rule: you can release freely available project but it can stil receive donations.
I wish to be able to release more and more free characters and monsters that can be used in commercial projects, but, unfortunately, don't have much time to...