He says the future of the time traveler's reality doesn't change. The Ancient One then reveals that a change to the past causes a new timeline to branch off.
ETA: Time travel in Marvel Comics has always worked like that. The best example I can give is of the seminal X-Men story, Days of Future Past, where Kitty Pryde travels back in time for the then-future (2013) where the mutants live under the dystopian thumb of the Sentinels. Future Kitty's warnings prevent "our" universe from following the same path, but it's shown later that the future for her still happened. Rachel Summers, the daughter of Cyclops from that future, ends up coming back in time and joining the contemporary X-Men.