That was the point of the resolution - ECE and DIY were excluded. Group == calendar event. Admittedly well strung out in some cases but also yo-yoing.
depends on the size of the field, I have ridden more than a few calendar events where I haven't seen another rider for several hours. When riding in a group I would always set to steady, but flash mode doubles the burn time of the light meaning it's much less likely to run out on me during the event. It's also more obvious to drivers who will be approaching at speed and not expecting to see a cyclist on the road at 1am.
(riding all night is mainly a problem for 400s, on 200s, 300s I would expect not much night time riding, and on 600s I would normally stop 2-3 hours after sunset and typically start again at dawn.)
If the event starts in the dark I would go with steady light, but after 250km the field is likely so strung out I would only expect to see other riders at controls, in which case maximum visibility trumps extremely unlikely discomfort to someone who probably won't be approaching from behind on another bike.