I've got a question about the rules: It says there "Any Art Assets you wish to submit must be available under an open license (I suggest the  CC BY 4.0)." But a lot of the existing submissions, including ones by Penrose who wrote the rules require purchase and run under licences I would not consider open.

Could we get a clarification what's allowed and what isn't?

Hi! I assume by "Penrose" you mean me, Penflower? :D
Submissions don't have to be free to be considered open license, but they must be available for adaptation and re-use in both personal and commercial projects.
I have gone through all the submissions again and removed those that did not fit the above criteria.
Thanks!