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 :)
While their account privacy policy is commendable, it isn't a proof for not collecting logs.
Even without personal details you can collect quite a lot of data - ip address that uses certain VPN account, which servers it talks to while using vpn, at what hours, at what intervals, also all the plaintext data exchanged between client and server. A lot of data that someone (who might already have your ip address mapped to your personal info, for example your ISP, or an online store where you shopped something before you turned on the VPN) would be willing to pay good money for. And companies like money.
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"
