This is a strange logic, since Redot is almost compatible with Godot, so if you open a Redot project in Godot or vice versa, tweak it a little, the player will not notice the difference, imho. How do you check that it is 100% made on Redot & not on Godot?
That is a strange logic to imply it's a strange logic. It is a game jam rule, and like many standard jam rules, it's difficult to verify.
For example, how do you check that the game was actually made within the allotted time? Really, you can't! If you use Godot rather than Redot and/or submit something that was made prior the jam, you've broken the established rule(s) and at the end of the day, the judges likely won’t be able to disprove it.
Best to be honest and self-improve via the challenge, rather than challenge the parameters of the challenge. If it’s not your cup of tea, I assume there are other jams on the roster, here on itch, that may speak louder to you! Good luck and happy jammin'!