The cluster thing is obviously hard to remove. But the population.. that should be very doable. You'll just have to adjust the people you chose according to the general population. Not sure exactly how that goes down in practice. But in the press conference he stressed multiple times how this was representative and was done in cooperation with statisticians etc.
Yeah well but is that really important? If they nail the same sample age-wise etc and use that to calculate lethality, then that number can be used on the general population, right? The only difference left is the rate of infection. But how many of those died should be the same.
I would guess that the spread of the disease and the rate of infection could be parameters to consider in planning policy for the immediate future. On the other hand, could this result be used to usefully estimate the total values in other regions from the hospitalization numbers?