Hello David,
I think they have not started working on the flow frames because when we got the colony (about a week before we did this first spring inspection), it came on 6 frames. At least one of them still had room for them to work on. And we added two new frames with only wax foundation. As you can see in the video, they started to work on those as well, but they are not finished, yet.
In other words, they've still got plenty of room in the brood box to work, before (hopefully) expanding to the super.
I'm not sure that they are indeed preparing to swarm. The couple of queen cups that we saw might very well be so called "play cups". I did not have the chance to check the hive anew, given that the weather has been a bit on the cold side, this last week. So, I do not know if the queen/play cups have been expanding into queen cells. I'm looking forward do do another inspection, though!