Not everyone is vaccinated and unfortunately the group least likely to be vaccinated are the group who are most likely to die of COVID - the oldest. The previously very low COVID rates in China mean that most people don't have antibodies. It's still a largely immune-naive population.
Hong Kong gave us a glimpse of how quickly it can happen. It's worth noting that some people doubt the accuracy of the death rate stats - is likely they were underreported as it was an advantage to people to do that.
And they had a more highly vaccinated pollution than mainland China.
China's spending on healthcare is relatively low and it's thought that acute care could be quickly overwhelmed, pushing death rates higher.
China has kicked the can down the road, and people are getting tired.