Hey there, sorry for the slightly long silence - needed to wait for the other judges to be around to give their opinion.
In the end, our decision is actually fairly simple: We can't really expect people to make 100% of a game's assets themselves, so using pre-made assets is fine as long as they are from freely accessible sources which are credited! Basically, as long as the assets aren't from a paid pack or especially made for you, it is perfectly fine to use them - anyone else could do the same, that's our logic.
Hope this helped clear up the confusion!