Private payments are pretty cut and dry nowadays, so hopefully authentication is fine. In fact, Brave sync being shared secret based instead of email based was one of the reasons I picked them up initially (although I now use platform default browsers via syncthing based sync).
As for piracy, I think that is partially why they made it free on Linux, and paid for others who are more likely to pay. I would hope the honour system works and people who can, do support the tools they use though. I would pay for this for people in my family who use Brave, for what it’s worth, especially as a one time payment.