Like can I try to make it be realistic? or does it have to be low poly?
I would say do whatever you want. As long as you learn the techniques in the course then it doesn't really matter how you apply them to your final result. Just have fun!
I define style as any non-realistic interpretation of reality: Cartoony, anime-ish, abstract, impressionistic, low-poly, minimalism, etc. These represent artistic expressions of reality rather than reality itself. Recreating reality is a great skill (most of what I do is realistic) but I argue that it's not a "style".
The exercise is based on style, measuring whether or not the submitter can achieve the same style as the example or apply their own style. So if you are to submit a realistic forest you will be docked points based on the stylized requirement. I know that may sound strict but we have to maintain an objective standard or the grade become meaningless.
That said, you're more than welcome to do a realistic forest if you want, like Matthew suggests! I completely respect artists focusing their time and effort on exactly what they want to do 👍