Hi Steven,
It's a nice question. As it is a server action, every time whenever this action is required to call, its output parameter will have their default value at first. So if you don't assign MemberAdded=False in the false branch, it will still return you false, if the current user is not HR Manager.
They have given it just for the understanding purpose considering the learner is a beginner :). You can try by removing this assignment.
Thanks and Happy Learning!!