A projectile launched by a mower cannot exceed the velocity of the blade. I calculate the tip velocity of a 24" blade to be 376 fps at 3600 rpm. To equal the speed of a .357 Magnum (1400 fps), the motor would have to turn 13,400 rpm. Three times that would rival a dentist's drill. That would certainly attract attention
Actually, the report says kinetic energy, not velocity. A railroad spike weighs about 2 pounds. the energy = mv^2/2/32 , or about 4400 ft-lbs. The blade is going to slow down a lot hitting something that heavy, so this figure is too high. We would have to know the mass and center of momentum for the blade (about 1/3rd the distance from the hub). In an elastic collision (i.e., nothing bent), momentum is conserved, not energy. A lighter object, like a nail, will not slow the blade much, but the energy would be proportional to its mass. They seemed to be mowing at fast rate/speed.
Sorry to spoil this party, but it was fun while it lasted.
Pouring nails into an inverted mower - which way would the nails go?