"For this specific game jam, you will need to use the Unity game engine because that is what UModeler supports." So you have to use Unity. However, you can make the game play like it's 2D, so as long as the visuals are 3D, you should be good to go. In fact, the post from FutureInspireGames (the other host) says "don't make your '2D platformer' 2D and flat. Make it at an angle so we can appreciate the 3D dimension" so 2D gameplay is explicitly allowed. You just need the visuals to be 3D.

In short, Godot is not allowed for this jam (only Unity is) but the game can be coded like it's 2D as it only has to be 3D visually.