Rule 5 requires a proper lookout. It does not require a separate lookout in addition to the helmsman.
You are correct. It does not specify a separate lookout or even that the lookout be a person but does specify that the lookout should be able to make a full appraisal of the situation and of the risk of collision. So if you are the only one acting as a helm&lookout and in a collision it's not hard to argue that as a lookout you were not able to make a full appraisal of the situation since you were distracted with helm duty. The rules are deliberately not specific, but they try to make sure everyone is at fault.