From the research paper, it seems that this conclusion is based on less creditworthy cardholders.
> The credit card products offered by the fintech company range from $500 to $10,000 in credit limit and 10% to 30% APR, and are generally targeted toward consumers with lower credit scores and/or shorter credit records compared with the general population of cardholders.
Who are exactly the target of most of the abuse in the credit industry.