One of the driving factors behind the bad mortgages that were issued pre-2008 was the lack of ongoing oversight and poor risk assessment with regards to those mortgages, driven primarily by securitization.
Your linked article itself tells that Geithner and his ilk admit to securitization being at the heart of the crisis, saying that "other factors were more important in causing the 2008 crisis, such as bad mortgage underwriting, poor work by the ratings agencies and a securitization market gone crazy."
But strangely, they write this as a defense of Glass-Steagall's repeal, as if those factors came from nowhere. In fact, those factors were the specific result of the efforts by the largest banks to sell as many of their banking products as possible--and offload as much of their risk as possible--through their investment banking and securities divisions. Many suspect that some banks took advantage of the inherent opacity of mortgage-backed-securities to knowingly hide bad assets (and possibly even fraudulent, non-existant assets) and sell them to unsuspecting securities buyers at fraudulently inflated prices.
Would securitization have become such a problem if the repeal of Glass Steagall hadn't effectively pushed all of the money center banks to acquire or build their own investment banks? Would it have been so easy for bad actors to have committed fraud in this area if it wasn't so easy to keep everything "in house"?
The main reason that Glass-Steagal was enacted in 1933 was because the Crash of 1929 was in-part caused by Banks' selling over-hyped, over-priced securities to ordinary, unsophisticated banking consumers. OK, so it happened in the reverse this time. That doesn't mean that the crisis wouldn't have been prevented by the same firewall, had it existed, from 2003-2006.
Yes, none of the institutions that failed initially were deposit taking. But maybe that was because this time around they were the outsiders and the rubes. Who had a better sense of what kind of risk mortgage-backed-securities actually represented, than the people who were originating them?