As Smart as Warren is, he's able to "understand" tech. What they do, why, etc. But what he is not comfortable with is the moat. How long will this tech company be on top. In tech it's much easier to be a disrupter than it is with Railroads, Utilities, etc.
Sure, Warren may understand Facebook, but who can confidently say that in 5-10 years they will remain dominant. With AAPL, it's a very sticky product and folks will not give up their iPhone, so he became comfortable investing knowing he has a bit of a runway to do so.
Cheers!