I don’t believe it either. It would have to be a thermonuclear detonation at that altitude. Remember, the electromagnetic wave decreases with the square of the distance.
It is not an EM emission from the nuclear device itself which causes EMP.
Rather, gamma rays emitted during the detonation cause Compton scattering in the upper atmosphere (stripping electrons off of atoms and molecules in the air).
The earth's magnetic field then accelerates these electrons to near-relativistic speeds; the moving electrons create large magnetic fields, which induce short but intense currents in electrical materials, leading to all kinds of fun.