Have you tried? A properly written Win32 app is supposed to handle drag-and-drop of files onto its window, and treat it as "Open". Try this in Notepad, for example.
That said, I'm not sure what this has to do with repurposing Windows Explorer? It uses the same widget to browse files, but the File Open window is not actually an Explorer window.
GP is talking about drag-and-drop of a folder into the file picker view, which results in the file picker navigating to that folder. Or drag-and-drop a file into the file picker view, which results in the file picker navigating to the enclosing folder and then selecting the file being dragged.
This is even more useful when saving files than opening files.
I see. I have to say, though, as described, this sounds as a rather inconsistent behavior to me. Does the macOS file picker not provide file management functionality at all (e.g. creating folders or moving them around)? If it does, it's kinda strange that it would behave differently from the standard file manager.
That said, I'm not sure what this has to do with repurposing Windows Explorer? It uses the same widget to browse files, but the File Open window is not actually an Explorer window.