Whoever hired him way back when was a genius. From optimizing the OpenGL pipeline using LLVM through creating a new compiler, debugger and static analyzer in clang, lldb, clang-sa all the way to creating a whole new language with Swift as well as forward-thinking, Bret Victor-inspired concepts like Playgrounds, he contributed so much.
And now he's leaving.
Possibly concerning, but arguably not that different from, say, Bertrand Serlet moving on back in the day. Talent like that is hard to retain in perpetuity.