There are two answers to the question. Either you “want an internal hire” because that motivates everyone working for you to work harder now, and you don’t actually care about the results that much when you’re not the CEO, or you say the next leader needs to be a visionary change agent regardless of internal or external, which heavily implies external.
But Apple doesn’t need a visionary leader at their size. They need a competent game manager like Tim Cook. So that’s why he would say “internal” but also why I don’t think this has that much implication on who they eventually pick.