Maybe, hopefully, people have been sick and recovered in those apartment blocs and not been counted.
All we can do is presume they know something enough to warrant the response they've had. IMO, a 3% mortality rate would not do this.
There’s probably a lot they don’t know as you say, but I think the crematoriums have a better idea of true mortality rate. Compare flu and other COD from a year ago with their current numbers...
One thing stands out- nothing but a grave situation would cause China to take the measures they have. No country did this for SARS, Ebola MERS, or the worst flus.
Any updates on these locked up apartment buildings? How are they getting food and med supplies?