Not really. Sometimes game jams give prizes for allowing the organisers to move the ownership of the game/idea/code to themselves. This jam pairs people into relationship of the "game design" or "game idea" and the developer. Either it changes anything in the result, is a different story. However, it is usually a good practice to put everything clear before start working on any project. It is just a suggestion to put such a note, that the game developer will have the only ownership upon the product/trademarks he made during the jam, at the same time, taking responsibility of any resulting consequences of the produced software (it doesn't have to be a time bomb, simply some copyright violations might be enough). Maybe it is already stated in itchio policy? Let me know.
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.