Most game jams have this same standard rule, especially with prizes on the line. As this game jam doesn't allow pre-made assets, how could any prevent someone from using a Kenny's tileset or similar, trying to skip out on work and saving time, not only using unoriginal art but also, copying and pasting code they do not understand. These type participants usual end up complaining about why their game doesn't score higher than they assumed it would. These game jams reward the ones trying their best, not the ones trying to show off, or steal a prize.
To answer your question, If you label your grass in the credits of your gameas "This grass asset I made originally for BLANK, reused to save time", that looks 1,000,000x's better than trying to sneak it in, and it will be forgiven. It is the honesty that goes a long way in this community. Second, is trying your best to actually learn something. Copy and pasting is not learning.
If you want the practice and want to simply recycle old code and assets, but still get the same treatment with videos and one on one feedback, I would strongly recommend the Practice Circle attached to out game jams, as a great place to practice and without the hassle of rules. There is also an optional theme to follow if you choose.
Good luck. I hope I was clear enough for you.