I think it’s a regulatory thing. PayPal aren’t your average BNPL product. Theirs is as regulated as a credit card (it’s a feature of PayPal Credit), and so you can’t make repayments using credit cards. It’s not allowed IIRC.
Think of it as less like Amazon installments and Klarna, and more like Monzo Flex. A fully regulated credit product with BNPL-esque functionality.
The unregulated BNPL providers do typically let you repay with a credit card, which might be responsible for the confusion. But that will likely end as they become regulated too.