I would think barely here means, that there were quite a lot of incidents and with some bad luck, none of the astronauts would have come home.
It was still a great accomplishment, so I would not use barely, but technically it is correct I think. They spend great effort, to just make it to the moon, which was the goal, to beat the sowjets.
And that succeeded. No great safety margins, not much room for error - barely.
What I meant is that the multi-stage Moon rocket, the capsule and command module, and the lander and ascent module, were just barely within the realm of the possible as dictated by the rocket equation. And, as you mention, the risks were just barely acceptable.
It was still a great accomplishment, so I would not use barely, but technically it is correct I think. They spend great effort, to just make it to the moon, which was the goal, to beat the sowjets. And that succeeded. No great safety margins, not much room for error - barely.