"Seventy-eight years, if I were to be exact. That was when Thorin and his company of dwarves last walked through Mirkwood and disturbed us."

This isn't something he'd lie about and, though he speaks confusingly, he clearly tries to hide nothing. He doesn't want Thranduil so angry with him but he's not going to make excuses. The truth is enough.

"Less than a year ago, Aragorn brought a creature to us. Gollum. When he escaped, you sent me to Rivendell."