Zwift has hard time limits for some of the laps on course (for example, the Volcano lap). If you go faster than those time limits you get the “Uh oh” cone regardless of how many watts you were doing. A group full of A+ riders can end up riding so fast that they all go faster than the lap limit.
I remember one series of races a year or two back where everyone would get an aero power up each time the crossed the line on the Volcano lap. Not surprisingly, giving everyone aero power-ups led to some very fast lap times which triggered a few cones.
I’ve seen the video for that race and the last lap seems to have been done in 11:13 which for a 9.3km lap which is 49.7km/h. I think you guys must have been right on the time limit which is why some people may not have been coned if they were a second or two slower on the lap.