That falls under "a certain amount of fraud going on". Yes, there were a lot of bogus AAA ratings going around. But everyone knew they were bogus - you don't get AAA paper that pays a 10% return. Any investment banker from the GFC era who tries to tell you they're a poor innocent rube who was taken advantage of by a dastardly ratings agency is having you on. The people buying the CDOs were sophisticated professionals who did their own analysis and bragged about how smart they were.