Meanwhile Amex have just launched this exact model in the UK called "Plan It", though it costs a fixed fee per month of the loan.
In other words, it's just a completely different model from retroactive, cardholder-paid flexible payment options for credit/charge card holders.
If that were the case I can see this being like the third party BNPL schemes such as Klarna sure. Otherwise it’s just semantics of where the option lives.