No, I still think the tensioner is shot. if you tighten the 19mm bolt and you can grab on to the smooth pulley and move it around without much resistance, the tensioner is shot. The nut is probably stripped because the cam on the back of the tensioner, which the threaded rod attaches to, hit a bolt head behind the tensioner and then wouldn't move any more. You probably kept turning anyways (which I did also a few times when I was new at this) and stripped the nut. Usually the adjusting cam will just break, but not always.