It could be just because your way of Tanking it is easier for rDPS, that does not mean another Tanks way of doing it isn't just as easy for mDPS. I have done it with range and melee dps as a healer and different tanks and I think it depends on the players more than it depends on their class. I have watched mdps stand in AOEs, but I have watched rdps do the same thing. I will say I do like to have at least one trooper when I am healing, not because they take less damage, but because they have Mortar Volley for when all the adds decide to attack the healer. Of course that still just depends on the player. Most PUGs don't ever look at the healers health bar falling like a rock while they are getting hit by adds and the tank is taking damage so they are busy healing the tank.
I understand wat you are saying but there are several ways to go with this boss and all of them presented more of a challenge for the mdps than the rdps. I repeat, certainly not in a way "not doable with mdps" but more challenge. And I am talking with my PUG experiences when defending my opinion on this. You are talking about a hypothetical scenario where everyone is fully aware of their surroundings and health bars as well as mechanics. Of course the fight is rather easy (just has a dps check for columi level players) if you execute the tactics flawlessly and everyone is aware and focused. But most of the time you won't have those ideal circumstances in a PuG environment, just as you have mentioned in a few examples. The player factor remains the most important one in determining difficulty. I merely explain you my overall experience. I right now rolfstomp this content every week with pugs too -- and that annoys me, but it has not made me forget the strife I've had getting through the first boss when I had worse gear and experience.