And if a game is exclusive on another platform doesn't that mean it's not on Microsoft's.
I had trouble understanding what you meant by this because the question-inverted word order doesn't appear to match the lack of a question mark. Did you mean "that doesn't mean..." or "doesn't that mean...?" So I'll try to reply to both interpretations:
Does being initially exclusive to Microsoft's platform (Windows) disqualify a game for a later release on Microsoft's other platform (Xbox One) once the game's developer eventually qualifies for the ID@Xbox program? If so, why should it?
The 1987 video game Contra by Konami was originally exclusive to a non-Microsoft arcade platform. In 1988 it was ported to the Nintendo Entertainment System, another non-Microsoft platform. Yet it got a port to Xbox Live Arcade on Xbox 360 in 2006. Did it get a pass because of the 1989 port to MSX2, a Japanese platform that shipped with Microsoft BASIC?