You can create a new node only if you are owner of the room or you are promoted to have host role i.e. role = 100 . When you autopromote someone, that user gets promoted to a role = 50 i.e. a presenter role. If you do not autopromote , the default role = 10 , i.e. that of a viewer.
Hence you need to promote the incoming user further to a role = 100, merely autopromote won't give him enough permission to create a new node.