The clavicle screams “Break me! Break me!”. It has no protection to speak of; there’s just a thin layer of skin between it and the outside world. Additionally, both ends of the bone are fairly immobile. A downward strike to the clavicle is (mostly) transmitted to the spinal column by way of the sternum, scapula, and ribs. The spine is specifically configured to oppose this type of compressive force and provides exactly the kind of resistance needed to make such a strike effective.
I’m not sure how these alleged medical sources tested the clavicle to it’s breaking point. 7 pounds per square inch could, if it’s applied in the most effective manner, break a bone the size of a clavicle. But who’s clavicle? An NFL defensive tackle? Probably not. A 130 lb bookworm? Probably. An average medical cadaver from India? Possibly.
The pointy end of a six pound blacksmith sledge hammer
wouldn’t require much speed to crack a bone. While the bones of a forearm are free to swing away from a 7 psi strike, the clavicle is supported at both ends and held ridged by the chest and spine.