The official fertility rates for that period was 1.3. For reference: 2.1 is the replacement rate.
If anything their total population went down during one child policy.
Even if I take your numbers at face value, it is absolutely possible for this math to math. To simplify massively, if the average person dies at 80 years old, the population growth today depends on the number of births 80 years ago, compared to today. Not 30 years ago. The population may have grown massively between 30 and 80 years ago, so that the absolute number of births remains high, despite a low birth rate.
And this fits for China where the standard of living has massively increased. What would throw off most Americans is that in 1962 the average life expectancy in China was only 50 years old, and has increased to roughly 78 today. 28 additional years of life is huge and it was so rapid that it would create a massive increase in population.
This also reverses causality on the one child population rule. They didn't add the rule because their population was huge at the time, it was added because increased life expectancy with nothing else would have increased their population now to something like 1.7 to 2 billion.