I'm not sure this is a huge caveat. This basically just says that if you're using a marketplace that you can plug Stripe details into to take card payments, PayPal isn't going to work. That's to be expected as that would be something the marketplace provider such as Shopify/Squarespace would need to implement.

Most marketplaces are not Shopify or Squarespace, and most customers on those few are probably already able to take PayPal payments via their marketplace where appropriate.