I bet he could tamp them in so solid they would not move an inch. Took me a long time to learn how to do it right.
We used a piece of oilfield sucker rod to tamp. Still have it.
Grandpa had a heavy metal bar with one end pointed. That end was used to help break up the soil for digging the hole, and the other end was for tamping. Kick in a few inches of dirt and tamp and then repeat until the hole was full.
Father-in-law used 4" or 5" round posts. Drop in the post and half fill the rest of the hole and tamp with the non-business end of the post hole diggers for just a bit. Then kick in the rest of the dirt and tamp once more.
Ha! Grandpa used 12V fence chargers and would test the circuit when done by grabbing the wire and then putting a finger into the dirt. That's how I learned to do that. Only did that once at Father-in-law's because they used 120V fence chargers.
Brother-in-law got a big kick out of it though.