Were Windows and Office ever part of the same org? I don't believe so - at least in this century. In fact, my understanding is that the antitrust consent decree required a separation between the OS and applications orgs.

Didn't Sinofsky go to Windows, after his rein in Office?

yes, but he didn't bring office with him to windows.

no, even before the antitrust fun they were always separate business divisions / orgs.