I don't have strong opinions on any of this, but that letter really reads weirdly. It has an almost-no-transition set of segues between "congrats for all the work you've done recently", "Sinofsky is leaving", and "so here is who's in charge now", with a conspicuous lack of explanation or connection between those points.
A former co-worker of mine used to occasionally bring up the first company email he received: "Effective immediately, so-and-so (not my coworker, someone from a different department) is no longer employed at <company name>. There will be no further comment on this matter."
