The apollo 1 issue was that 100% O2 was used in a capsule test at sea level pressure i.e. 1000mb of O2 in round numbers. One spark and the rest is history. They didn't stand a chance as the massive rise in internal pressure held the door shut against the outside air.
The atmospheric system in-flight was designed to supply O2 at about 200mb, about the same partial oxygen pressure as the air we breath at sea level. The result was that the capsules could be designed to be light as you could get away with extremely thin walls. The skin on the lunar module was so thin that it could be punctured by a biro. The acceleration forces experienced on the ascent from the moon would often buckle the airframe.