I'm pretty sure both Win32 and Qt are singlethreaded unless you (as the programmer) choose to spawn more threads, but I might be mistaken about that.

Well, modern QtQuick-based applications will be multithreaded by default because the update and render logic are divided between different threads. Of course that is only a limited help because any blocking update logic will still make the UI feel unresponsive. However, at least controls like for example the BusyIndicator will keep animating even with a blocked update-thread.