Well you can think what you want about Chinese asses. I personally believe about 90% have flat asses (even guys) but there are those that have nice round ones. I believe with the right clothes; such as the girl in the blue shorts the flat ass can be accentuated to look more prominent, after all THE CLOTHES MAKE THE MAN or woman in this case.
As for the photo you refer to, I saw that in Shanghaiist.com in I think in 2015, it was a take from WEIBO (Chinese Internet) about a group of about 20 scantily clad girls in black underwear and high hills that took to the streets to advertise a smartphone social media app called "What's inside" the second line reads "use me", needless to say they were stopped by the police and criticized by every Chinese newspaper and internet site BUT the promo work, The app got high publicity.