The conclusions of this analysis match my own personal experiences discovering that friends and others I had success with in business were also Amex cardholders. Amex cardholders are generally more affluent and better at understanding and handling their own finances compared to cardholders of other brands. I chalk that up mostly to Amex being more selective about who they issue cards to.
As an investor that means there's a lower risk (or higher risk adjusted return) for every dollar Amex holds on its books compared to other card issuing lenders.