In Jam rules, there’s a point:

You can use Unity to create the game, and any assets you’d like, so long as you credit them.

Does this mean game should be made in Unity, or is this just a recommendation?

It is more so just a suggestion than a requirement. Whatever you choose to use, Unity, Unreal, GoDot, or even Game Maker, just make sure that if you use someone's work for the project from an asset, you give them some credits in the description. Just make sure to include ANTs as mentioned in the description.