You are correct. It is always prudent to have a legal disclaimer.
Prize jams usually have a larger "legal" bit where they clarify what they want from you and save their asses form all sorts of consequences. The would be lawyer inside me would argue that since you never waived any rights when joining the jam (and you were never asked that you read the terms and agree to them or that you are old enough to do so) you retain full ownership.
I remember reading the itch policy many moons ago and it said that you have to have the legal rights to use anything you upload. So that part is already covered. I am also positive that uploading malicious software is against the tos. Never organized a jam, but I am sure that they also have an additional tos with even more stuff, but I am currently not in the mood researching that.