Have you ever looked at the list of the songs she has written? It is incredible.
Yes, it's such a long list of big hits. And she can perform most of them herself. The life-version she does of "Diamonds" is stunning, I like it even better than Rihanna. But I'm a sucker for Sia's voice.
The best act is when you can convince yourself that you're not acting. And since most people learn how to act/fake from early age, they get really good at it and believe themselves to be the act.
The examples you bring up are true for every generation. They're made all the time, by everyone, and even crossing generations - "Don't mind grandpa being a racist, things were different back then." Excuses make life easier superficially, but do bring a ton of complications in the long term.