As far as I've seen it, that response from him was in cases where people tried to simply use the rate of infection measured in Gangelt and apply that to the entirety of Germany. That is obviously invalid as Gangelt had a much larger outbreak than most places.
That doesn't mean that you can't learn anything from this study for the rest of the country, just that you can't extrapolate it naively.
The thing is that it's simply not exactly stated what kind of population they tested. Whether they were representative of that town or the German population. Of their sample was actually representative of the German population (which I heavily assume since they talked a lot about that) then the results will be quite robust.
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?
Takeaway: death rate is lower than many initially feared. But it is not like we have already 15 % immunity. In reality it is probably around ~1-2% based on the number of deaths.
That doesn't mean that you can't learn anything from this study for the rest of the country, just that you can't extrapolate it naively.