I believe he was a military historian and based his books on a commission report on this topic that came out simultaneously with the 9/11 commission report and subsequently was buried from the public consciousness due to that horrific attack and public attention.

That report however pointed out the general serious vulnerability of our electrical grid infrastructure and painted a fairly bleak picture of the potential effect of such an attack.

Forstchen, may have taken some literary license but in scanning the report (its online) perhaps not. Anyway, he tells a story well.