Checked. Went directly under the naxx (in video the person flew directly in it. I missed it a bit and went under into the "mushrooms"), then did /target Anub'Rekhan and was able to. Had a mage friend invis and run towards the boss, and then pull him (from "normal" naxx, as if he was in a raid) - and it was the same boss that i was targeting.
Therefore:
1) Very very VERY nice find exploration-wise
2) Unfortunately its not different zone/id/etc, its the actual physical location of two instance parts. Basically, [ x @@@ y]. where X is naxx entrance, @@@ is mountains etc, y is the "northrend" that you can see from KT's chamber, and KT's chamber itself. Its kinda same as in Ulduar teleport, and ICC teleport, etc. The locations "seem" near each other, but they are actually a long "Physical" distance away.