The game doesn't think you're in a romantic relationship with Derek when he wants to be where you are. This story is about Derek and prioritizing him. Whether it's platonic or romantic, the goal is to have him become a much bigger part of your life and to no longer have to be apart so much like you have been the past few years. I'm sorry that was more than you wanted, though it is the intended point of the story that Derek wants to be part of the MC's life and will make the effort/leap to try doing so. If you don't want him to be a really, really significant person to you, it is better not to play his epilogue instead of Cove's.
But the part about telling people you're together or not does sound like a bug. That line is meant to be only for the romantic version of the story. I'm sorry for that confusion with that!
Basically though, people who love Derek get to make him special. We didn't want to make those who cared about him disappointed by making it feel less meaningful than what you could have with Cove when doing Cove's story.