Yes, that's a big if regarding EMP. A terrorist scenario will likely not be a high altitude detonation. Then again, a different terrorist scenario would be an EMP attack using a device specifically created for that, like a high power microwave-type weapon, but that's another topic.

I've read that about modern cars, too, although I don't know if anyone has actually tried to test that out in one of the few facillities that can simulate an EMP. So, that may just be an educated guess by someone.