I don't doubt that the people selected to walk on the moon were indeed fit and healthy, but it seems like a stretch to think that the people who flew on the earlier Apollo missions were selected to less stringent criteria. The selection effect should be just as strong for the people who "merely" got to fly around the moon. So out of two sets of people selected for being healthy, one group seems to be much more alive than the other.

(Also yes obviously the sample size is too low to draw meaningful conclusions)