I assume it is and use Medium as a blog/diary. Medium is what I would call a Social Writing Community. It is not a literary magazine, online publishing journal, poetry/prose online magazine. Anything you post here is absolutely yours. You own all rights to it. Someone please correct me if I am wrong.
If you have something you want to submit that is currently hosted in a publication here, simply communicate that with the Editor (s) of that pub. I am sure they will understand and will be excited to see you take on that venture of submitting that piece for this contest. Good luck! 💙 You will blow them outta the water!