I'm not so sure on some of the assumptions (for example, Apple are currently using Bing as one of Siri's search engines), but the general thrust of why Google would be acting defensively around Windows Phone does make logical sense, at least when viewed with a broad brush.
As the world is switching to mobile, Google wants to secure their position as market leader in advertising in mobile as they are on desktop. Mobile advertising dollars are the future of Google's earnings, and who's their threat? Appe and Samsung do not have a search engine or a remotely rival advertising network... But you know who has a Search Engine, and Advertising Network and has a Mobile Operating System, right? It starts with an M.
Now that Microsoft have their own smartphone hardware division, they'll be a brighter point on Google's threat radar. Depending where you measure it, Windows Phone's market share is on the order of 3%, 5%, or 11%. Not enough to seriously damage Google's income stream, but enough that they could in the future.
Is that why Gmail, YouTube, and other services from Mountain View aren't as friendly on WP as they are on Android?
That's Google's main reason to not support Windows Phone. Apps like Chrome, an official YouTube App, Google Maps, Drive, Gmail, etc, only will make Windows Phone more appealing to end users.
There's a lot of big moves going on with smartphone strategy at the moment, do you agree with PostPC's reading of the situation?