I think djohnston's point was that China's global reach started expanding after they stopped killing tens of millions of their own citizens, which was around the time Mao died.

Well the USSR used over 30 million people as slave labor in their gulags, which were mostly involved in construction works and natural resources extraction.

That can certainly advance things.

> ...too busy purging tens of millions of their own citizens to develop the global reach...

I think you misread 'to' in this sentence to mean 'in order to'. The intent by the parent comment was 'too busy...instead of develop[ing] the global reach'