I've done 2k+ dps solo tanking zodiac many times, and there are plenty better tanks than me. I've seen numerous tanks do better or at least approach 2k.
I've also seen it done on Strom, if you count him as a boss. DTs have done 3k damage solo tanking Arbanus. DTs often top the parse on fights like the Imp, zodiac, and the Bat, if they aren't tanking, and sometimes even if they are. Before they fixed the bug in basilisk, I could easily solo tank him doing way over 2k dps, hitting long stretches over 4K, and finish the encounter with most dps, dmg taken, and healing. Someone once posted a basilisk parse in here where they were hitting 5k solo tanking him with a DT. 3 tali spec'd DTs can melt the general with no hope of mages pulling agro except another DT.
If you don't see these sorts of things in your raids then it's because your DTs aren't spec'd to do it, don't know how, or are told not to. IMO, it's silly not to have your DTs run that way on certain encounters. Once a DT gets rolling with the right spec, the Zodiac won't kill them unless the puzzle gets botched or the boss gets enraged.
DT is one of my main toons, but I won't and no one can make the argument they aren't ridiculously OP.