Memorizing raps doesnt = incredible rapper to me. Jay-Z was incredible to me for other reasons, none of which are this writing in his head thing. Maybe if he put pen to paper he would be way better than he is, lyrically.
But not writing means not reading. Jay doesn't SOUND LIKE he's reading because he isn't and that helps the delivery. Gives it a better feel.
Also it wasn't just memorization but rather HOW FAST he came up with it. The one take nature of it.
It takes pure raw natural intelligence in addition to conditioning to do what he did. This is a natural talent developed into a useful skill and I think he should be applauded for it.
At the same time though I think people calling it "a freestyle" is stupid and the exact opposite of what it is. Raps being prepared is tantamount to them being written even without them being physically written or typed.
People know damn well people prepare "freestyles" they spit on radio (or for a time rap city) and for pretty much ALL battles but when you see someone like Meek reading his Cassidy diss off a phone or Canibus pull out a notepad in the Dizaster battle people don't respect it and for good reason.
Being able to do verses in one take and not fumble at all or having to punch in and out requires intense focus and great breath control.
I know Jay said he forgot perhaps multiple albums worth of content through this process. I also know Jay don't remember all his lyrics. I remember his forgetting an entire verse of imaginary playa and repeating one verse twice. Heard he messed up on Renegade too a few years back but I never saw that.