It's really difficult to get to that meaning, because "main thread" is what you call the thread where main() is invoked and which is where all the GUI calls must happen. You can't use a common term in context to mean something different from its common meaning and expect people to follow.

Thank you for this. I see now how my wording was unclear, but this is exactly what I had meant. Sorry for the confusing word choice.

Changing a fundamentally well-defined term (main thread, i.e. the one on which main() is called which is running the core event loop for the application process) to be an incorrect opposite is rather fundamental when one is explaining an iOS best practice.

except on iOS GUI thread kinda has "main" attached to it, main_queue