I don't think the 15-minute thing is really significant. It's a nice round number but after thinking a bit I doubt they went through the effort to script something out to be fifteen minutes long when they all read through it. And if you edit a video randomly or stop a stream at a random time, the odds of its length being exactly X minutes and zero seconds are only one in sixty.
I do think it was pre-recorded, though, because they stressed so much that what they had said had been pre-approved by managers and lawyers. If they really wanted to make sure the livers were saying the right things, having that vetting happen on a pre-recorded video is best, to avoid the chance that somebody riffs on a live stream and says something they shouldn't.
Now if only those lawyers and managers would pay attention to what their talents were tweeting on the 20th.