True the dps were doing it wrong but you have to wonder what the tank was doing as well, they should at least be building enough aggro on the whole pack to prevent the healer pulling.
I've run instances with him before, both as a healer and as a dps in everything up to hard mode content. Tank is his alt, and he was not every well geared at the time, however I've never had issues.
Even if the fault was
entirely on him, it's besides the point. Pulling a "look at the guild tag" as their only response is incredibly silly and tacky. If you want to be respected, act respectfully towards others, don't demand it because your guild tag is well known.
And they did in fact do what he asked, and everything went fine after that. I can also say that there is only so much threat and so many taunts a tank has in a large group of mobs. If dps are just letting 5 weak mobs fire away while trying to hold the strongs and elites, they will eventually turn on the healer. DPS can practically 1-2 shot them, they should always die first.