Apple is ridiculously "undervalued" compared to any peer in the industry. It is a money printing colossus.
Having said that, the market seems to get shivers around $600B, and that psychological barrier imposes a friction that makes the rules of the game change. No one can stay above that for long, and in the case of Apple its profoundly out of scale numbers makes everyone simply put it in a different universe of valuation.
For instance, no one thinks that Apple has a big vault with hundreds of billions in it. Like all rational players, they hold most of their negotiables in short term securities. For tax reasons they also have been taking on debt in some areas rather than liberate cash from elsewhere.
But they have an enormous, enormous amount of wealth sitting virtually at a standstill. They have announced that they're going to spend a lot of it to buy back shares (boosting the value of the remaining shares), but they've been doing this so slowly they're adding to their pile quicker than they're spending it.