The official contest rules are here [0]. There is actually very little in the rules restricting how the image is created; technically it doesn't even say it has to be a photograph, although that is clearly implied. The rules mostly cover usage and legal rights.
So the artist intentionally exploited a lack of clarity around AI in the rules to gain publicity. This seems clever in that I might not have heard about this artist or this contest otherwise, but it was a cheeky move to win a photography contest by playing a meta game.
0: https://www.worldphoto.org/sony-world-photography-awards/ope...