Maps worked fine with exactly the same Trident engine on the desktop. Moreover, if you change the user agent string, maps also worked fine with Windows Phone.
That seems like a pretty simple fix for Microsoft to make on behalf of every Windows Phone user... Google can't compel MS to identify its phones for Google's benefit.
Again, there's no rule that says devices must have unique user-agents. Once you've established that Google wants to block Windows Phone from using maps, you can send the user-agent string of something they don't want to block.