I won't make a generalization like "old growth is the problem," since I view such things on an individual case basis, but I would say that many old growth forests are at risk largely due to accumulations of undergrowth.
My understanding is that the HFI was largely written by the Wilderness Society. IMHO, when considering the magnitude of our nation's forestry problems, it's stupid, it's expensive, it's bureaucratic, it's too little, and too late... but it is a start. At least they have acknowledged that there is a problem. They don't understand the causes much less the solutions. I think the longer term is much more serious than we realize.
I've stayed out of the day to day fire stuff because I find most people's understanding of what needs to be done so upsetting. I have put up one little rant you might find useful. Have you dug very far into the book?