I am not convinced that all of those that has been submitted for the contest is already can be called as "intellectual" property of roobet. Participants were just ofcourse allowed to used Roobet related theme to the artwork. Not because you submitted an artwork for the contest, means that they own your artwork. Maybe the issue that could come here is the fact that the artwork that @Maus0728 contains an identity of roobet, which I think is needed to be discussed and allowed by Roobet before he can use it to his NFT artwork.
It is a vice versa situation and there is a competition rule as well. For example if roobet actually ended up asking for it, and you give it to them, it's theirs and they can prove that in a court. At the same time they do not even have to use it but ban you from using it as well if they want to. However we are in crypto world, do you really thinking that some 5th in an art competition for roobet will not be able to use their own art because roobet would claim it's theirs?
I do not think so, in fact I am pretty sure if that guy used Roobet logo as NFT and sold it, that would still not create any problems, Roobet may not want that, after all it's their own logo used to gain money, but who would really work enough to figure out who that person is and where they are and which nations and sue them for it, that's way too much fork over some NFT and that's why I do not think it will matter, it's just good manners to ask that's all.