No, it’s kt. Fission weapons have a much higher E1 pulse than the fusion weapon used in Starfish.
Knocking out all the cars’ computers in the country along with all the telecomm would not make for a fun month. Food would flow...slowly.
I'm a former missilier, we studied and planned for these things. Regardless of what your book says, 10kt won't do squat at 400 miles up as you said. To damage the whole USA’s telecomm, electric grids, etc they would need multiple 20MT explosions placed across the country, or as we said, maybe that was the Soviet's intended use for their single 100MT weapon. (It wasn't much good for anything else!)
This is a new bugaboo for the sky-is-falling sensationalists who've apparently just learned about EMPs. Just as nukes themselves made those types crazy, so too the EMPs will “Destroy the world”! Sorry, nukes won't destroy the world and neither will EMPs.
EMPs are something that can be, and are, defended against. Effective shielding can be as little as ordinary aluminum foil wrapped completely around an electronic device. The military has more extensive hardening systems for our critical assets. Yeah, electricity and phones will be gone for the citizenry, but that's what backup generators and stored, shielded radios are for. As for vehicles, replace the circuit boards and they're right as rain.
You might have to learn to live a nineteenth century lifestyle for a few months, but that's hardly the end of the world!