wdym bought? it was never a game you needed to pay for up until the release of episode 9
I'm guessing that they may have downloaded the game via browser (or not with the big red install button while on the itch app) and supported the game while it was free by giving some additional contributions?? If downloading on browser, a pop-up does show the optional "This game is free but the developer accepts your support by letting you pay what you think is fair for the game." choice, and doing so likely made itch think that you bought the game whether or not it was free, depending on how much was contributed at the time.
That happened to me when I paid for the DLC expansion on A Date With Death when the main game itself is free.