“No such thing as Robin Hood.”
The Doctor had been so sure he was right and that Clara would be disappointed, but the figure before them dressed in Lincoln green certainly looks a lot like the Robin Hood of legend. Not that the Doctor has any intention of believing it. He must be an illusion, or an impostor, or something. Robin Hood isn’t real and nothing will convince the Doctor otherwise.
Being both right and wrong at the same time is interesting; this Robin isn’t a real person, he’s a robot.
But that doesn’t prevent him from being a hero.