I don't think the inertia of the blades have anything to do with breaking the pin, rather the inertia of the motor/pto continuing to drive the shaft after the blades have seized. Same principle though.

I'm still at a loss on why he's waiting for a ride? Disconnect the tractor from the equipment, drive the tractor back... Heck, I'd think even once the PTO shaft was disconnected, he'd still be able to tow the equipment back using the tractor.

EDIT: Of course, I'd guess it would take a few tools to disconnect the equipment so it could drive back, or tow... so perhaps that's what he's waiting for, tools, not really just a ride, and then tow the tractor and equipment back...

He says he's "done for the day" because of it. Probably intends to get a new shear pin "tomorrow" and continue mowing. Depending on how far away everything is, it could be more efficient to avoid two extra tractor trips just to repair a pin.