No doubt Chinese take learning math(s) seriously, but not mentioning the red flag in the PISA study misrepresents the statistics big time!
See those two asterisks? You're looking at the most prosperous elite cities in China. And while Singapore has many Chinese and a confucian tradition, it is also a small prosperous city. These examples are not exactly representative!
I hear that the big thing in China right now vis-a-vis math(s), is learning from south asians who also do very well in STEM.