That’s the most likely theory as far as I can tell: While a credit card doesn’t make it more likely to get the money back in case the customer does run away with the car, it’s probably less likely for high-risk customers to even have a credit card.
Which unfortunately creates all kinds of false-positives, like making it very hard for tourists from countries where credit cards are not commonly used to rent a car…
It’s probably to avoid lending the car to higher risk demographics.
It’s a test.
The rental car company basically loses fewer cars overall and thus has lower insurance premiums, and thus can offer cars cheaper than other rental companies that do not require a credit card (if any even still exist?) that have higher premiums to pay.