1) Machine Learning is one of the most quickly advancing fields of learning today, maybe ever. Having discovered a lot ten years ago likely isn't as important as what you have done now. The importance of the machine learning people that do AMAs on reddit (Lacune etc) is that they lead teams of people now who are working with streams of big data now, publishing state of the art results now. Which brings the second point...
2) Machine learning has advanced to its situation by being an incredibly open field, with arxive being the default repository for publications. Just doing this hire doesn't give any indication that Apple has relaxed its posture of restricting all information coming out of its research. This closed-to-publication position makes them much more unattractive to cutting-edge machine learning researchers than the presence of a single big name researcher could make them attractive.
(and I'm no cutting edge researcher myself, just an observer but I think you can find similar comments from real researchers in places)