That's considering a perfect scenario where you kill the boss in 2 phase 1, means no damage on the raid, or almost.
if you do a 3rd phase 1, for example the boss is at 20% and you gotta do the same once more, there is no way at all the raid will be toped starting 3rd phase 1.
All healers will have aggro 1 by 1, killing them, and the boss will move all the time making it impossible to tank, nature clouds will be in the middle of everyone, and at the end, there is no healer alive because the tank can't catch up 10k heal aggro quick enough.
Viscidus had a shit ton of rescripts since aq did open, first week it was dead within first phase, then they implemented a script making it impossible to one shot in 15 sec, then the blubs did move faster. Nowadays, you gota do the blub slow mode, and freeze the boss to death (means perfect scenario boss dies when freeze 2nd phase 1).
Perfect scenario don't really exist in a progression guild, so I recommend the tanks using as much threat as possible (without sacrificing to much ehp, obviously) to be able to grab it back once entering a 3rd phase 1 where generally a wipe is due to heal aggro, or a down because the tank has enough TPS.