You assume the bombers were definitely carrying nuclear weapons. They needn't if your goal is to have the enemy wipe itself out by this method, they only need to think you are carrying them.
Also, I argue that your line-of-sight is increased by "low-altitude" EMP blasts (30 000 ft for a localized EMP on a bomber vs >100 000 ft for high-altitude EMP) whereas a nuclear detonation probably occurs at much lower altitude (~2 000 ft), so the higher altitude could in theory attack a much larger area as far as power is concerned. So in terms of electrical infrastructure, a larger area of damage could occur by Nike attacks on bombers higher up. But who knows.
Also, I argue that your line-of-sight is increased by "low-altitude" EMP blasts (30 000 ft for a localized EMP on a bomber vs >100 000 ft for high-altitude EMP) whereas a nuclear detonation probably occurs at much lower altitude (~2 000 ft), so the higher altitude could in theory attack a much larger area as far as power is concerned. So in terms of electrical infrastructure, a larger area of damage could occur by Nike attacks on bombers higher up. But who knows.