Like, he can be a 10/10 tank in all aspects, except dps and threat generation. Maybe that is also why he is good? Because he is ignoring the dps part of his tanking job.
Very possible he is a good tank. However to be a great tank you also need to dps well.
There are examples on this even in top tier guilds. Just as an example fraggoji from paragon when he used to play protection paladin. He probably used his defensive cooldowns well, positioned himself well, moved from shit, but his rotation was abysmal.
Like watching this video from his PoV
made me shiver being a prot paladin myself seeing spell usage that bad. I can understand why he rerolled.
Point is, some tanks can be awesome tanks but terrible at dpsing as a tank. When a a tank your jobs are.
Help raid and cotank as much as you can with cds and stuff
Pick up mobs
Being 1/10 in DPS and 10/10 in the other categories you can still seem like a great tank, but you can notice it when a tank loses aggro that he is not 100% sure what he is doing. And as said before, one of the reason a tank can seem good is that is completely ignoring his rotation responsbilites allowing him more effort to focus into the other parts. Its easier to joggle 4 balls than 5.