Weren't we number 1 in season 6 too with that daft frost strike sigil from ulduar?
DK's were absolutly top tier in season 6, and throughout all of it. DK/Rdruid, DK/Hpala in 2's also got r1 on quite a few BGs. They were also pretty beasty in s9 with TSG and Hpala/2xDK
The problem with DK's has and will always be that Ferals and Warriors have five times more uptime on their targets and are always super good in the same seasons as DK's.