I would think the pump drive would shear pretty easy if there was junk stopping it, I have seen them sheer off in motors with 20W50 people had to start below zero.
This wasn't a cold start motor, it was running when this happened. The peice could have been just large enough to make the noise, still pass through the pump but had enough drag to stop the engine. Obviously the engine started again and the metal may have been crushed enough to still be in the pump and exited later. I am thinking with logic and logic sometimes flat won't work in real life.