1970 was during the cultural revolution. In that year, the world population was 3.68 billion, and the population if China was just shy of 830 million - China had 22% of the world's population, so if they held (only) 20% of the world's prisoners, they'd have a lower than average incarceration rate.

The same is not true for the US today, we have less than 5% of the world's population today.