Start by the fact that in their system you are just a random number that is generated at the moment you arrive in their website. They make very easy to pay without having to use any of your personal data, like Credit Card. Overall I think if a company put so much effort on that side, they simply don't have any data to log, or if they do is pretty anonymous anyway :)
In Brazil the central bank has such system for free, it's called PIX and is so good and simple to use that some fintechs lost a good chunk of their market share. You only need your National ID or an email. It became even an expression "send me a pix"