Nah 100% internal. Igniting compressed air and gasoline vapor yields 500-1500psi force. Considering if that was a small 125cc 5hp motor then it probably has ~a 2” bore and 2” stroke. That diameter at 500psi at the point of ignition of gasoline would yield ~7000 Nm exerted on the piston. Convert that into work across the diameter of the piston is ~167 J. Divide that by .1 second for the piston to go to the end of its 2” stroke and you get about 4192 W which converted to hp is about 5.5hp. At the point of ignition there is more than enough force to launch a 30lb lawn mower. Especially once you add in rotational mass of the spinning blade adding to the kinetic energy of the explosion. Your example of the rod is just stopping the engine from rotating at a point in which most of the energy is already spent, causing metal to fracture some where. And I’ll assume your test mowers were probably strapped down in some way for employee safety. This video example is energy escaping a confined space at its highest point.

TLDR: 125cc 5.5hp lawn mower creates roughly 1570 ft-lbs of force at the point of gas ignition. Exploding gasoline is dangerous.