What a fascinating undertaking. I wonder how it plays with the backwards compatibility of "newer" powermacs on macos 9
This reminds me of a quite Mac centric BBS system called Firstclass. Its client felt almost like a normal Mac OS Classic Finder windows and it was so intuitive compared to normal text based BBSes which I also used at the time.
 I remember seeing it working with Windows 3.1 or Windows 95 but it felt more kludgy and slow compared to the Mac version  https://en.wikipedia.org/wiki/FirstClass
Somewhat surprised that you could even use threads in System 6. I know in the 7 era you'd find applications that shipped with the Thread Manager, I'm guessing this individual must have rolled their own thread handling?