There is always a Captain. When you start a game you are the Captain by default, and who fills the roles on your ship is up to you (you, the AI, or friends). For that reason I wanted to differentiate between the Captain, and the four roles under him/her.
We are not creating Artemis, with five linear positions that must be filled. The Captain can be the only human player on a ship, and so it has a special status above the other four roles.
The text is directed at the player as the Captain, and so listing the role of Captain under that would be confusing I think.
I am open to your thoughts on this, I just wanted to lay out my position on it.