that event took place around 50 years before the quest for erebor in the books. -So that would explain why Gandalf was shocked with the news of a necromancer in dol-guldur.
Because, unlike Tolkien's Middle-earth, here he did not discover the Necromancer in Dol Guldur at that time (nor had the Necromancer's existance been revealed yet). 'There are older and fouler things than Orcs in the deep places of the world.' - Gandalf the Grey, The Fellowship of the Ring