The study methods are closer to advocacy than science or policy.
Sure, take any slice of a vast number, and you get a big number.
It's not a "wealth transfer" when everyone gets what they bargained for and can opt in or out.
Most importantly, the transaction value of using credit cards or rewards systems - what the user actually gets - is not enumerated.
Beyond what others have noted (mainly deferred payment), credit cards offer legal transaction protections: my legal liability for fraud is limited (unlike debit or Zelle transfers), and I can challenge any transaction even later, which gives the vendor an incentive to ensure I'm happy even after they have my money. While reputation provides some incentive for repeat customers, the ability to retract a transaction governs even non-repeats. I would submit this alone has improves quality of service for everyone anywhere credit cards are accepted.
Rewards vary by type. Cash-back rewards reflect the fact that interchange fees were set to recapture initial investments, but servicing costs have plummeting (thanks to computing); governance-wise, it's almost impossible for a "representative" political system to extract a large cost from a small number of powerful agents with vested interests to provide a tiny amount of benefit to a very large numbers of other people. But that's a much more extensive governance issue.
So where does the benefit go? To competition between credit providers, initially as cash-back, and then to tying rebates to future purchases within controlled channels. For airline point systems that give free flights or upgrades, it improves retention, but other forms of rewards would seem to verge on tying, where power in one market is extended into another.
Politically-mediated wealth transfers are a political issue. Economically-mediated wealth transfers should raise market-regulation policy issues, in particular whether the law is inducing or protecting them, and then whether they are good or bad. Tallying that requires not just seeing the money flow, but seeing all the value received or cost exported.