you can't prove what was made during the jam, unless you make everyone in the jam live stream their screens, then who's going to watch it?
"You are free to use premade or store assets provided you have the license for them."
lets face it, "asset" these days means premade game.
I think it doesn't matter tho, because of the theme and the judging are very specific, so even if you start from a pre made game, the resulting game will be different.