I have got two mates who can perform a one arm pullup (one of them only using the last two knuckles on his middle and ring finger) Both are super advanced world class rock climbers.
As far as resistance training goes they mainly involve hang and campus board training, the odd bit of forearm and shoulder work and heaps of core stability/ab work. They are also as flexible as a 14 year old russian gymnast! The rest of the time it is climbing and bouldering.
Both of them are very light (60 and 68 kg each) and have the lowest BF% I have seen!
When performing a one arm chin up they appear to ‘twist’ their way up. Starting in a semi pronated handposition and finishing in a supinated fully locked elbow position. legs are used for counterbalance one out to the front and one to the back.
I have a feeling that body weight has a lot to do with being able to execute a one arm pullup. I reckon that the heavier you are (even if it is mostly muscle) there is a disproportionate increase in difficulty. Especially if you are evenly distributed muscle wise (rock climbers have scrawny chicken legs)