I am guessing they are saying that other classes will lose a lot more than Shamans/Enh BECAUSE of what you say above. Namely, with shit legendaries and shit set bonuses, they are getting very little contribution to their numbers currently. So when removed, they will drop very little, versus another class that might get a ton from legendaries and set bonuses that will drop a lot that might drop them below Enh on the rankings.
Realistically, Blizzard seems to only have 2 truths about new expansions and that is that Warlocks and Shaman suck. I expect they will "fix" these rankings so ensure that Shaman remain at the bottom, because they seem to have it out for Shamans, though I cannot understand why.