Christina usually works on the songs with Sia though, or at least she has in every single one they did together even if to alter them. So I don't see Sia writing a song giving it to someone else and then handing it to Christina.
This is really old, but seems believable! It's why Katy Perry and Max get along so well writing wise, they are both analytic and each word has to fit a certain count in each line. Bonnie McKee explained this process too in an interview.
The swedish pop rule is all throughout Katy's work (with Max) and Kesha too. 1989 follows the same pattern and so does Queen of the Clouds by Tove Lo. It makes for the catchiest, relatable, and melodic songs to grace the airwaves
I love both, amazing artists, please, make more songs together! 
