I wouldn't be so sure of that. My sister, a dual UK/AUS national, came over from Australia a couple of weeks ago. Her UK passport had expired and she had organised for a new one to be sent to me in the UK so that she could pick it up when she was over here. So she did have a valid UK passport, just not physically with her. She took her Australian passport with her to Brisbane airport...and was denied boarding because she didn't have an ETA. She was told by the airline to apply for an ETA online, but she couldn't do so because you cannot get an ETA as a British National (the site explicitly asks). She had to fly back home, get a one time emergency UK travel document from the British Embassy, fly back to Brisbane a couple of days later and then board the plane (she was very lucky they let her move her flight with only a nominal fee).

This is the reality of the new ETA system. If you are a dual national and you only have your other passport, you cannot use it to enter as you won't have an ETA (and can't get one), and airlines will deny you boarding if you try boarding with a foreign passport that doesn't have an ETA...