To what end? There is no benefit to locking down 2.7M people beyond possible reduction of transmission. That “control” offers no benefit to those issuing the order.
Why do you assume they're thinking rationally when they're acting irrationally?
My guess is it's a desperate, irrational attempt to save face and appear strong. The chinese government is extremely anxious/insecure about their image.
From the beginning, many (including much of the US media) have praised china for their alleged ability to contain the virus early on.
The reality is they were just arresting people who were reporting the truth and were censoring their death count as of April 2020.