There is people who work in there for more than 15 years and are still going strong.
Clearly the decision wasn't his, but from a higher up.
You haven't been reading the right blue posts. GC himself as well as at least a couple of CMs have said that job descriptions at Blizzard aren't as rigid as they are at other companies, and they have a fairly flat hierarchy. I reserve the right to maintain that 'Lead Designer' doesn't mean 'person who makes all the decisions'.
He didn't say he's leaving Blizzard, he said he's switching projects.
Clearly the decision was delivered by the people working over him. Or under him. Or his family. Or the cosmos twitched and his alphabet soup spelled "Time to leave the D3 team" in his alphabet soup yesterday evening. Clearly.