Source?

I am honestly curious. I know for example that the US was very dirty in the early 1900s, but we had 100 million people. If China was dirty in the 2000s with 1.3 billion people, it stands to reason that China was more dirty, no?

Technology has helped curb the absurd pollution that was the product of older processes. We don't have data from industrialization in the US, because we didn't have the tools to measure precisely the pollution at that time. But based on verbal descriptions of the pollution, China today might be similar to the US when it was industrializing. Specifically, historians note the frequency of days where pollution is so bad you can't see across the street, which used to happen in the US regularly yet infrequently, which is the same narrative description you'd see in China today.

Surely it's the opposite. China pollutes far less per capita than the US.

Less per capita can still work out to more in absolute terms.

It doesn't make it any less dangerous for public health.

Postulating that this was a necessary step to industrialization might require some strong evidence.