I can possibly see it if poor gasoline caused the engine to 'diesel' or run backwards after it was turned off.
I think the excess use of starting fluid can certainly damage an engine, but I can't for the life of my own engine see how it could shear that pin. There is too much other mass in the way that would have to move too quickly - the piston pushing the crank turning the cam (at 1/2 the speed) & then still have enough left to shear that pin . . . . ? ? ?
I can see broken rings, pistons & cranks, cracked heads or cylinders, bent connecting rods, or blown head gaskets. but not that.
But I have been surprised before & hope to again - just not soon!