What they are trying to say is this: You will be judged on the work you did during the gamejam. This means you if you send in a game you have been working on for 3 months already, it will only be judged on the work done in the last week (during the jam). You are however allowed to use assets from the store without penalty. Since this is usually just animations and models which are things that take time to create, so they are allowed for use in the jam. It is something else entirely to have 3 months of coding and testing done already, hence the penalty. I hope this clarifies it 