I don't think it's a scam at all. There is a game jam with similar condition for winner. It called IGMC 2018 host by Degica. The jam is done well. I don't see any problem happened there to the winner.
While they're not the same company but both associated with Maker Series. KADOKAWA is the company that developed both RPG Maker MV and Pixel Game Maker MV. There is no reason to not trust them.
If you worry that much , you don't have to submit the game that you think it gonna make you a billion dollar! (if it ever make that much lol) You can come with new fresh idea whatever you want.
This is the game jam. Just have fun! Let's make the game.