Read all three pages here and everyone mostly discuss the reputation. I think this is the least interesting part of the clear as you can clearly farm it in silithus with a dedicated enough group of players. How many hours it took is not interesting either as theoretically the entire raid could get the rep before even setting foot in the instance which means they would only need to get essences once.
What IS interesting however, and I have not seen this discussed, is that in order to obtain essences you first need to kill four different types of trashmobs. Once this is done you go back to Azshara where you get the quest to kill Lucifron, Gehennas, Shazzrah and Sulfuron.
In the video they go straight to Lucifron after pulling the first mob without stopping. The only way to do this would be if they already cleared the required trash in a previous raid as I dont see how they would have had enough time to go to Azshara before Lucifron in the video.
Now while I am sure they did this legit, it would be nice to see some video from the raid where the trash quest was done. It would certainly put this debate to rest.