This is about an OS-layer API, not a windowing toolkit API. Tying the window to a thread at this layer makes it impossible to have a single-threaded UI application. It is inflexible.
What does that even mean? Of course you can have single-threaded UI applications in Windows, the majority of them are. Have you ever written a Windows application in C or C++?
And yeah, i would stop signing your posts - it comes across as conceited somehow; like each comment is a big deal.