6 feet was chosen as a matter of policy because it was the most you could reasonably get people to stay apart. Sidewalks, store aisles, elevators, etc, are all about 6 feet wide and so it's a good number to convince people to cross the street or wait for the aisle to clear. 100 ft would be a much safer number but that's just not really physically reasonable (and so the guidance would have been dismissed by the public).
The choir practice should not be an example of how easy it is to spread; rather, it should be an example of the unreasonable intuitions people have about 'distance'.
Remaining in close contact, indoors, while singing, for a long period of time is not a good idea. It is quite reasonable to expect that to lead to transmission.
Similarly, two people passing by each other on a sidewalk, with nose breathing and mouths closed, even if it's closer than 6 ft., is a very small risk. Still good to maximize space, but nothing to get worked up about.
I don't have a source, but a microbiologist I know was saying that safe distances, based on droplet physics, depend on activity, e.g. standing in a single file line vs walking vs hiking vs running.
Regardless, doesn't matter. Singing/shouting/chanting with a high density of people inside a closed setting is dangerous for any repository disease as viral concentration just keeps building up. Same thing happened with Shincheonji on a far larger scale.
"Members of the choir were already aware of the COVID-19 outbreak in their state, particularly around the Seattle area, which is about an hour south of Skagit County by car. Hand sanitizer was dispensed at the door of the practice that night in March, no one shared sheet music, and people were conscious not to stand too close to one another or engage in their huggy greetings. Also, no one remembers anyone coughing or sneezing, and no one present felt ill at the time. Nonetheless, a huge percentage of people present later tested positive for the virus."
My point is that the original article said that it's hard to get infected. Assuming that everyone was asymptomatic, and there was no coughing or sneezing, and everyone stayed away from each other, it's actually not that hard to spread the infection. In fact, if there was a single person who was asymptomatic and they infected 45 people, that means it's very easy to get infected.
It may very well be that it's very easy to spread this virus, but I don't think this incident is a good indication of that. It seems more an indication of how poorly even well-intentioned people understood the contagiousness and what exactly social distancing meant at the time this happened (1 month ago).
Brings up the difference between public health risk and personal risk. Public health perspective you want contacts to be infrequent and importantly short.
That is this evidence doesn't contradict Streeck arguing that formate transmission is unlikely and that it is long term exposure to breathing people that is the real risk. (Or that being in a relatively empty grocery store provides little risk)
(I don't have a clue one way or the other, just commenting on applicability of evidence)
Standing 6 feet away from someone for an hour... Is another story.
Also worth noting: My wife has worked with a lot of community choral groups. She is convinced that they took all reasonable precautions, but doubts that their rehearsal space was sufficient to accommodate everyone standing 6 feet apart from eachother for the entire exercise... Or even for any part of it.
I leave it as an exercise to the reader to work out what this means for the typical tech firm's open office layout.
Also this is self report of behavior after people did what was in retrospect a very stupid and risky activity.