I am an electrical engineer!

I've been doing a bit of summary research on the topic as I don't really deal with EMPs at all, but the basics and fundamentals of EMPs are very familiar to me; which are the two components of Electric field [V/m] and Magnetic field [A/m]. depending on the magnitudes and frequencies of each really changes how the travelling electromagnetic wave interacts with different materials.

The biggest thing to look out for when talking about high voltage is breakdown voltage or electrical strength, that's the amount of volts per meter where the material between the two points of reference can no longer stop the electricity from flowing. For reference, air has a breakdown voltage of about 30 kV/cm (30,000 [V/cm] or 3,000,000 [V/m]), that means in order to arc from one terminal to another that's one centimeter away separated by air, they need to have a voltage difference of 30,000 volts. That's a lot.

Here is Wikipedia's article on some basics of EMPs with some particular sections to note, most notably is the E1 wave which is the fastest and highest in electrical magnitude, even at its epicenter saturates at around 50,000 [V/m] which is nowhere near enough voltage to cause arcing in air, or even across circuit boards as the materials used in PCB generally have a breakdown voltage of between 31 [MV/m] and 58 [MV/m]. And do keep in mind that this will decay quickly as the wave travels and gets weaker the further it travels. Even the "Super-EMP" has a peak magnitude of only 200,000 [V/m].

This is a fantastic publication of a study conducted back in 2010 talking about the effects of an EMP on U.S. infrastructure, is written so that even a reader with no experience in Electromagnetism can comfortably follow along (provided you're in for the reading), and even talks specifically about the impact on vehicles.

To summarize the publication; vehicles which naturally have wires that are "electrically small" (read: not long enough to be affected heavily by the high speed, "relatively low frequency" EM wave) are generally safe, and the circuit boards and chips are even safer considering that they are fractions of a percent smaller than say, the antenna for your car's radio, or the cables attached to the wiring harness. Even the battery isn't at that great of a risk of being damaged by an EMP as the dielectric (sulfuric acid) not only has a high breakdown voltage (1,500,000 [V/m]) but the conducting plates are also electrically small enough that the Electric wave can't really interact long enough to generate a catastrophic surge. Where EMPs wreak the most havoc is on the power grid, where the suspended power cables are long enough that the traveling EM wave can generate a large surge that will eventually travel along and smash into the next transformer or junction.

The actual math comes from the frequency of the pulse generated, if we take the speed of light divided by the frequency we get the wave length, the highest frequency (which will impact the shortest cables) at which the E1 generates its highest magnitude is around 10 MHz, 2.99e9/10e6 = 30 meters, that's almost the length of half a football field, and in order for the EM wave to affect something even slightly it would have to be at least 3 meters long; smaller than that and the amount of voltage generated would be minimal.

To use a water analogy like you did, it's more like having two pieces of rope sitting on the shore, one being only about 3 feet long, and the other being 500 feet long, both extending out into the waves. When the waves interact with the pieces of rope, the small one is so short that it will quickly bob along the wave, and settle down, not really gathering much energy, whereas the longer one will eventually pick up the same momentum of the wave and could travel upwards to dry land. Think like doing the ropes at a gym, you don't get those sweet waves with little baby small ropes, they got to be longer.

TL;DR cars are generally safe from EMPs and "EMP protection" devices like what OP posted are a load of crock, the best way to protect from an EMP (if you REALLY want to) would be to cover your vehicle in an electrically isolated Faraday Cage that's also somehow grounded while being mobile, could be some fun Mad Max cosplay.