That is what I meant when I said taking things the wrong way. I'm guessing Blizzard meant the Pantheon couldn't kill him and not they just left him, otherwise he would be running rampant and basically owning Azeroth. Since the lore says "the 3 remaining Old Gods were imprisoned" we can assume he is chained. 3 remaining Old Gods being because C'Thun was presumed dead and they destroyed Y'Shaarj, this means Yogg, the unknown other one, and N'Zoth were chained.
"N'Zoth was imprisoned by the Titans under what is now the Great Sea" is from WoWpedia
http://wowpedia.org/N'Zoth
I'm pretty sure that's what Blizzard meant ^^