They could have gotten it anywhere.
There is no evidence they all got it at rehearsal.
They may have. But it’s no fact.
Coincidence only stretches so far.
Sixty singers, 45 of them sick, 3 hospitalized 2 dead. It is mathematically impossible for a group of 60 to have 75% infected when a random group of people would have less than 1% infected unless they were all infected by the same event.