Yeah, I've read that book. It's interesting that Greenspan managed to make everyone forget he was an acolyte of Rand's right about the time he shot to fame. I've also heard the claims of Greenspan's massive (negative) influence on the west. So if your argument is that Rand (and libertarianism) influenced Greenspan, who influenced everything else (especially the bubble, which he partially caused), then I agree with you. But that's an extremely roundabout way to have influence.
Because Rand was only one of presumably dozens of influences on Greenspan's political makeup, and Greenspan was only one of several causes of the 2008 collapse. And keep in mind that fringe beliefs can cause major occurrences and still be considered fringe. (Also as I said below to u/sphericalvoxel, fringe to describe libertarians is somewhat hyperbolic.)