Alan is being quite a bit disingenuous in his explanation.
Firstly, he's cleverly utilizing a logical fallacy that assumes that the bubble can only have one cause, either a low funds rate or low mortgage rates. Because, he argues, low mortgage rates are, ultimately, to blame, it's not the low funds rate, so it's not his fault. In reality, looseness of money was due to both phenomena occurring and, had Alan paid attention, he may have been able to actually do something.
Secondly, the Fed's mandate doesn't limit them to operating solely through the funds rate (as we have seen recently). Had Alan been properly motivated he would have possibly come up with ways of preventing the accumulation of excessive leverage.
In general, I think he's just playing games and trying to sweet-talk his way out of the responsibility. I don't buy it. It's not all his fault, but he should definitely take some of the blame.