Red flag is for danger to the recovery marshalls, or debris on the track, both didn't apply
There was time to reset the cars if the had a process to quickly reset the cars which is not hard, but they don't. Using the pitlane is an option for that, but no pitstop, let the lapped cars stay on the track and SC sits at the front of the pitlane, and exits with the cars when last lapped car passes the pitlane. Or slow all cars to a crawl, the lapped cars can pass all cars until they get to the lapped car they are behind. The leading unlapped car is at a crawl, the others can close the gaps