Wearing away the babbit down to the insert could have been a result of improper crank grind or out of spec bearing insert. Once the oil comes up the pump its main oil galley->main bearings->cam bearings.
If your cam bearings are good then oil got to the main bearings.
Here is a photo of the oil flow. Even if you ran the motor for a couple minutes(yeah I know, its not something you WANT to do, but running a motor for a bit with no oil is not a huge deal, and I know the 'po-fessional makaniks' are going to howl.. but it really isn't going to destroy a motor in seconds) as long as there was no real load not much would happen. The bearings were lubed (from your previous priming).
Since you scarred up a bearing, I'm going to bet that the motor had other issues. Machine shops screw up all the time, wrong parts get installed etc etc etc.
