How good of a conductor wood is depends on its state. If it is very dry and not salty, this should be safe (although he could have taken the piece of wood more at the end to increase the distance between him and the fence and the length=isolation through the piece of wood). If it is wet and salty, it might be dangerous.
There was a major storm just a few hours before this, so the wood is likely wet. However, he threw it against the fence; he wasn't touching it when it made contact with the wires.
The higher the voltage, the less the electricity cares how good a conductor something is. Air is a shitty conductor, and yet, lightning doesn't give a shit.