Let's assume the "real" share of infectious people in the population is 10/100k (That's twice as much as the most recent reported 7-day incidence (5.1/100k) of new infections for Leipzig, the area where the study was performed.)
Further let's assume a PCR Test has a 30% FNR, and a 10% FPR (numbers completely made up, I don't have a source).
Then out of 1500 People who have tested negative, we'd expect 0.05 to have the virus.
Once the expected value starts to creep over 1, then that's enough to infect quite a lot of people. It's not too much past that to expect that one of the infected has 'underlying condition' and risking death.
In British Coloumbia there's been a big outbreak due to a single gathering in the interior, notably (shamefully) attended by Medics and Doctors.
It's a little risky.
It's just not the time to have major gatherings, but at least this one is in the service of the science of 'stopping it' - which is better than most places are doing.