Wherever I look, everyone says that Tk/Tkinter has no facilities to call/send events into the main thread (even in Py3). So the only option is to poll some queue or something from it by repeatedly calling .after. And that mttkinter is the only real salvation.

So, stating that in Python 3.x, doing tkinter calls from another thread in not a problem as you do is quite a groundbreaking claim to make!
Do you have any references to back it with?