https://bugs.winehq.org/show_bug.cgi?id=45713
--- Comment #8 from Stellarspace macariopatrick@icloud.com --- The launcher installs the version of Unity Web Player it needs (via some sort of script) to the PREFIX when a user clicks the PLAY NOW button for the first time. I even reinstalled the custom version of Unity Web Player with the actual installer and it still displays that message.
Unity Web Player loads in Retro.exe and displays that information on the screen.