When I installed and serviced invisible fences I regularly ran into lightning damage like this but on smaller scales. I’ve found entire fence 2000’ perimeters were the copper was gone and the jacket still in tact but brittle and crumbled when you touched it. And I’ve found section of the ground blown out, usually where there was a splice. I’ve never found an entire line with the ground blown out, but I have no doubt it’s real. Look at the jacket that’s left. It’s burn and brittle. If anyone had dug that up it would have been busted into pieces from the shovel hitting it.