Matt--thanks, though personally I think that in addition to a strict OAC, a +bodyweight chin (or, preferably, pullup) is the 'gateway' number. And of course, some have gone far beyond that....
Center of gravity/weight placement definitely does matter. For example, it might be harder to hold a proper hollow position with a dumbbell between your legs; and I find that a vest/weight over shoulders setup restricts my scapula movement enough to decrease my best numbers. My best weighted chin was 1 rep with my younger brother (~120lb at the time) and he hung around my midsection--so the weight was in front, rather than below me, and did not restrict my arm/shoulder movement.
Obviously, strength is specific, and bodyweight strength moves will be harder for a larger person. Most strongmen probably prioritize heavy bent rows, curls and cheat curls, etc. to heavy chins and pullups; though many are quite simply strong enough to do chins (Derek poundstone comes to mind) while others do like weighted chins (I believe mariusz pudzianowski could do pullups for reps with +80kg or so). A swimmer, who probably is not carrying a lot of excess weight, has a good power to weight ratio, and does a lot of pulling/lat work in his general training is definitely going to be stronger at chins than most strongman competitors, who despite their lifting power might be carrying a bit of extra bodyfat and simply aren't as used to moving themselves around. Though again, the strongman probably has enough overall base body power that with a bit of practice, he could become quite a powerful chinner.
I've known people who were excellent at pullups but not at much else. Again, strength is specific, and upper body pulling strength has little correlation to, say, squatting. A rock climber might be incredible at chins and levers, but barely be able to full squat his own bodyweight if he has not done general strength work. Same goes for armwrestlers, etc.
Where this gets complicated is bodyweight... a really heavy guy who does bodyweight strength work is probably going to have some solid lifts too even without training; and a lighter fellow who lifts heavy might be very strong at bodyweight exercises with relatively little work on them. An example--I'm on the strongfirst deadlift team; during my prep for our meet this year I did no pullups. After not doing strict pullups for more than 2 months (nothing other than deadlifts, actually) I came back and did 20 strict pullups, 80% of my all time best, despite being seriously detrained at them. The absolute strength carried over due to my light weight.
As for my friend (really just someone I was in contact with via a forum), he was just a freak in general. As he said, his body responded unusually quickly to any training, the more strength-focused, the better. At a bodyweight of only 148lb, he deadlifted 340 the first time he tried the lift. In addition to banging out one arm, one finger chins for reps, he could do straddle planche pushups and a momentary maltese hold on the floor. Lifting was less impressive (other than the DL) but he could bench press 225, despite orangutan arms, and power clean and jerk 210 with minimal training. I wonder what he's up to... he had the most natural, raw athletic potential of anyone I've ever known.
As for assisted OAC: hanging on to your wrist (a one hand, rather than a one arm, chin) basically just challenges your grip. You are able to pull with both arms, and only have to hang with one. This is a valid progression, though--decreasing the leverage over time by moving your hand from your wrist to just below your elbow, then just above, then high on your bicep, then onto your shoulder--when you can do 2-3 strict chins with your 'free' hand on the shoulder of your working arm, an OAC should be well within reach.
The 'twist' is just part and parcel of one arm chin work and is a skill to be practiced and grooved like anything else. I set up at an angle, which greatly reduces twisting.
/Ramble, I hope that wasn't too hard to follow....