Seems to be across the board. My level 67 paladin (who has done the quest) can't talk to him in random BGs nor can my main when I can go in there solo.
I entered at 66 earlier, Found no one could talk to thrall, So i went and picked up the quests outside, COmpleted it in another run, Then back there again in the random queue 20 mins later and could talk to him again without anyone having the quest.