Sure a lot of "use to" or" back in the day" on here. Difference between pullups and chins, as we all know.
Seen a 150lb gymnast train with a 180lb weight harness, while doing pullups on a set of rings. Probably around 10 or so reps. Another guy doing a one arm chin with 70lbs on the waist, for reps. The free hand behind his back. He was a rock climber. Most of those men climbers, and women, have exceptional stamina when it comes to grip and pulling power.
I had heard of a Olympic lifter doing a bit over 300 for reps, but never seen so don't know how true it is. I have seen guys pulling up a couple of hundreds plus quite a few times in serious training. Not saying that a 400lbs lift is possible but not very likely. But I'm sure JF is just the man to do it.
Want to get stronger in pullups. than try 6Xof 3's.