Yeah I know that. Obviously, it wouldn't own itself. That doesn't make any sense. A small pool of insider investors, the management, would ultimately own it.
But they'd have to pay for it with their own money. They can't use Apple's cash to pay for Apple. When Apple buys its own shares back, it effectively increases the ownership percentage of its remaining investors -- who still own all of Apple. Spending all of Apple's cash on buybacks wouldn't fundamentally make it any cheaper for anyone to take Apple private.