That is the most nonsense I've heard in a while.
Although I was not at that Zero 7 show, I've seen Sia a number of times, mostly in LA where she can sell out multiple nights on her own, and she has never ever come across as an ego-maniac at all. She's just been silly and charming between soulful singing. Even when she was a surprise special guest singing back-up for Inara George, she only sang back-up and at no time tried to take over the song.
The room was mostly empty until right before Sia's set, and then it looked more than half full. This had absolutely no impact on the show.