Oh brother, I don't know where to even start with this one!
First off Nasser looks amazing there, he's 320 lbs and with abs in that video (from Nasser on the Way Part 3 shot in 2000).
He also clarifies that he's doing it just for the stretch, and isn't claiming to be able to do full range chinups with that kind of weight (400+ lbs considering that he's putting 90-135 lbs on on his 320 lb frame).
He also does that at the very end of a long intense back workout, in it he does many sets barbell rows, deadlifts, one-arm dumbell rows, seated cable rows, and pullovers...all before going to the chinup bar.
Finally, he's not a powerlifter or strength-trainer.