That has nothing to do with the point that a strike on one grounded system will raise the voltage on that entire system to hundreds of thousands of volts. Everything on that grounding circuit will have a huge voltage rise and fall while being discharged. If there is another grounding system on the same premises, the difference in voltages during a strike between the two will be huge if not electrically bonded together. If we properly bond the systems together, the entire system's voltage will rise and fall at the same time, effectively nulling out the voltage differential. The rise and fall thing works two ways. Not only is there a hazard when lightning strikes your radio equipment, but also when lightning strikes your electrical main's grounding system. When that happens the ground conductors from the electric supply will be thousands of volts higher than the metal connected to your separate radio equipment ground.