I think a lot of it is practice. He said he's been doing off top freestyle raps since he was 12. He's also a jazz drummer, so the rhythm, flow, and cadence part just comes second nature to him and he doesn't need to think about it, and he can focus on finding the words. And yes, very good memory indeed.

I watch a lot of his videos, but this one is quite next level, because there are so many nuances that he has to hit and he has to stay on topic. When he's given words to rap about, often times he go and say how dope he is or how bad other rappers are, which I think are his filler bars; he can't do that in this one. Having to stay on topic and hit each specific detail point given by the prompt is completely insane.