The pulse needs to be extremely powerful to even think of frying an off-line diesel generator. Even then, a generator can be quickly repaired.
Most of our testing so far to produce the best EMP is megaton-level nukes in space. Such a thing could take out most (not all) modern active, unshielded, non-hardened electronics in the US. For non-nuclear pulses, you’d have to get right up close to the power plant with a father big bomb because such pulses degrade rather quickly under the inverse-square law.
If you read the whole article the point is that without external electricity the spent fuel rods get hot, whether the lack of electricity is due from one cause or another.
At Fukushima there is no electricity due to earthquake and tsunami damage, but the point is the spent fuel rods not being covered with water plus no electric for water circulation and heat exchange.
And very possibly damage to the holding pool, and pumps.