Well, we have no evidence that Col. Christopher would have returned from his Saturn mission (in corporeal form anyway)... Or that the mission would have been considered a success. All Spock tells us is that the mission originally took place, although it is implied that it would be a good thing if the mission again took place in the amended timeline.
Really, we have to account for one additional oddity here: how come neither Kirk nor Spock originally remembers Col. Christopher's contribution to the history of Earth spaceflight? Granted that they never discuss the first man, woman or dog to have flown to space, to the Moon, or to Mars, either. But both do know an awful lot about the early DY vessels, enough to engage in one-upping banter about the finer points of that technology in the "Space Seed" teaser.
Is it just that space exploration became ho-hum after the Moon shot, and the people involved ceased to be deserving of historical immortality, until Cochrane discovered warp? Or was Christopher's contribution somehow lesser than the contributions of Gagarin, Armstrong, Carter, Hawke, Gordon, Dare, or whoever spearheaded mankind's conquest of the Sol system?
Perhaps Christopher would have been less noticeable if his mission turned out to be a failure. Then again, he could at least gain notoriety that way.
Or perhaps Christopher launched on the first Saturn probe, but more advanced ships flew past him to Saturn and back four and a half times before he actually arrived?