I have heard it was your group that came up with the rule of thumb. I know nothing about it! And frankly, these black walnuts we have make fruit akin to small cannon balls so I would not care to be under one if someone was giving it a thump unless I had a helmet.
I had a Gum tree in my front yard. They look harmless but if they hit you in the head, they leave holes in your scalp. Kind of like being hit with a nut mace !!
That tree played a part in my early teen years. It was skateboards as the new toy. Not the big polyurethane wheels that rolled over things but the metal skate wheels. I lost a lot of skin to that tree by the driveway.
That was my first thought. We have acres and acres of planted pines in the South. They stick these itty bitty trees in the ground in rows and just leave them. They don't even water them in.