The Doctor would have Clara believe there was never any such person as Robin Hood. He’s merely a legend, an invented character, a storybook hero. Clara doesn’t care; whether Robin Hood is real or fictional, Sherwood Forest is where she wants to go, so she can see for herself. She knows by now that all legends have a basis in fact. The Doctor’s a legend across space and time, and he’s perfectly real.
So is Robin, for all that he turns out to be a robot. In the end, what matters is that he existed, not that he never lived.