It literally says it on the sales page that you can’t.
Take your new-found powers out for a spin and discover what you’ve been missing. Your new level 60 character will come ready for battle outfitted in level appropriate gear. (Boosted characters are unable to enter dungeons and raids from previous expansions for 24 hours.)
I think the OP’s point is that WoD dungeons are not really previous expansion content when they’re a part of the current m+ pool. They were able to go into Karazhan for m+, but not into a WoD dungeon.