Yeah, the rules on assets are fully separate from the newbie category. As long as any assets you use are assets you have the right to use, you are fine. The newbie category simply requires that this is your first game jam.
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Rules on assets from the overview:
"Assets
You can use any art, audio, code, or tools that you have the legal right to, but you shouldn’t make assets ahead of time that would be specifically used for this game jam. If you previously made an asset that ends up being good to use for the jam, that’s totally fine. Just don’t do any “prep work”, heh.
You can use any assets available online! (Free assets, purchased asset packs, etc - if you can legally use it, it’s fair game!) I also request that you don’t use generative AI to produce art or music for your game. "