You can use pre-made assets.
> Does this rule just mean that you shouldn't slap random things together and call it a day? That if you're using assets, that you'd have to make sense of them?
Yes, that is exactly what it means and is intended for. If you use assets, make sure it's an appropriate one, and if the gameplay is polished you won't get penalized for using temporary graybox art instead of finding some asset pack.