Somehow I had gotten the notion that the Council was confused over whether the Necromancer was Sauron, or the Chief Nazgul/Witch-king of Angmar. Judging from the quotes from the chronology apparently not.
Actually, now that I've finally located my copy of The Silmarillion
, it does seem to say what the Tale of Years does not: that in the timeframe leading up to The Hobbit
it had been suspected that it was one of the Ringwraiths in Dol Guldur as opposed to Sauron himself.