According to the start stream GDTV did, there's no real penalty if you miss one or 2.
The idea is for both GDTV, and us as potential players/judges to be able to get an idea of how much of your own work is in the game.
There are some out there that will use premade everything, possibly even code and claim that they made it all themselves, but they're normally pretty easy to spot.
Outside of the jam, a lot of paid assets and some free ones will require attribution as part of their license to use the assets. If this is not done, the original creator could try to hold you to account, so it is also a good habit to have that if you didn't create it, credit it.
This is part of the reason why I always aim to make as much if my own assets as possible. More work, but it's all mine.