Bona nit!!
I think that you have reason about the fact that the normal (low altitude) nuclear explosions described in Twilight will produce an EMP which will affect only a small area.
About the shielding of vehicles, I suppose that is much easier due to the simple reason that any vehicle with a metallic enclosure (or fuselage) will work (with variable success) as a simple Faraday cage. That's the explanation that justifies that planes and cars are not affected by lightnings.
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