That may be true, and would perhaps be applicable to this case, if Eric were a manager. He is not. He is (was) a Principal SDE. This means he writes code and likely has lots of input into high level architectural/design decisions. Any influence he had over the actions of others is likely due more to respect for his technical acumen than any kind of managerial title. I suspect the closest he would come to being a manager would be any mentoring (official or unofficial) that he did, which is likely more effective than being someones official boss :)
It's not their particular authority over others but their general importance to the company's image and the company's importance to their image which incentivizes everyone to use euphemisms in the situation of a departing of manager. It seems logical that the case of a Principle SDE would be similar.
Not that managers don't leave for better things on a regular basis but making that the leaving-mantra sadly has reduced its credibility.
Edit: It would be "conspiracy theory" to light on one explanation and push it blindly without overwhelming evidence. But speculation in the dark is fine if it is labeled as such. Real, concrete information is better than speculation the dark. If one could count on such concrete information from Microsoft and others, one would want avoid.