Re-reading the page now it makes a lot more sense. Last night when trying to submit an entry I was very tired and easily confused and my brain lit up in panic when I saw the words "must [...] have been made in Unity". It probably would have been more clear to me what was being said if I had read the jam page more carefully in the first place. Sorry for making a fuss and thank you very much for extending the deadline as I had gone to bed without making the submission before seeing your response!