When i tried it decades ago (someone told me it was great) I sprayed it and had misfiring. Found out it conduct's electricity. Wiping off the excess and cleaning around the terminals inside distributor fixed it. Not sure why people spray plug wires or coils. If their bad they will arc and short, with or without spraying. Coils, no idea why someone would do that. 12 volts isn't high enough to have issues. Ever start a vehicle while the starter was submerged? I have a bunch of time's. It didn't care. If it was a problem you couldn't drive when it rained.
I bet the guy in the video did it because his father did it, Father did it cause his father did it.. none understood what they did just that someone told them it displaced water. Oh, WD40 is petroleum based, it will rot some types of rubber it gets sprayed on.
I never worried about my old carb vehicles or new ones when it came to water crossings. The old trick was to glue a small plastic tube to your distributor and hook a small vac hose to it. the water would evaporate and get sucked into the tube. Ran that on a few old jeeps i had. My Land Cruiser suffers from the same water intrusion in the distributor. If i get misfires from a deep crossing (or power wash) I just pop the cap, wipe it out and spray part wash in it and call it good.
Scott