In the adult/porn world, there's a high amount of chargebacks and fraud relative to low-risk industries like SaaS software. If you pass a certain chargeback threshold in the adult industry, your account is terminated, and no payment processor will do business with you.
To reduce the likelihood of passing that chargeback threshold and being banned, OnlyFans uses "cascading payments", which essentially load balances the payments across multiple payment processors in order to reduce their chargeback ratios across their merchant accounts.
The payment is either processed by Stripe, Securion, CCBill(the leading payment processor for adult), or another company.
Last time I checked the network requests, I noticed it was storing the card on Stripe, CCBill, and Securion, but using CCBill or Securion to process the payment.
I think Stripe is there for models on the site who don't sell adult content. OnlyFans probably does a check to see if the page is adult-related and if it is, then routes it to the correct payment processor.