19 one arm pull-ups

Pretty impressive. Years ago when I worked on trapeze I could do one with my dominant arm. I should really try it out again, but I weigh a lot more now, so I'd be basically starting from ground zero. I can get a rep using both hands & 20Kg. I use 16Kg for GTG (grease the groove) style multiple times daily training using 2 hands. I suppose if I keep getting heavier & heavier 2 hand chins, eventually it will help me develop the strength necessary to lift myself one handed. It's then just a technique issue (rather like a pistol - loads of people have the strength, but the muscle inhibition is the issue you have to overcome).
Anyway here's the video enjoy:

I think you make it sound easier to do than it is. I think a lot more people can bench press twice their bodyweight than can do one arm pull-ups.

Most pulldown machines, for example, only go to 250 lbs and most weightlifters would need to be able to pulldown around 400 pounds to do a one arm pullup. (Not saying you need a pulldown machine to train for this, just saying it's really not normal to be capable of that kind of strength.)

I don't think pulldowns work the muscles quite right. I remember reading a study ages ago (sorry I haven't got it to hand, it was several computers ago!) that implied that muscles work quite differently doing a pulldown to doing a chin up. I think where one the arm is moving & the other the body is moving makes the whole insertion & origin change around & so the muscle actually functions differently, you are also stabilising the body, so muscle can be inhibited more on a chin-up.
I was trying to make it sound as 'simple' as I could, not 'easy'. I suspect that working the stabilisers when using one arm puts it again into a different league, so you have one league with pulldown as you're very stable with just limbs moving. When you're ready you can move up a league & work towards a chin, which is much less stable as the body is moving, next up would be a one arm chin on a bar which adds a rotational/core element to the whole thing, so that's another league up from ordinary chin-ups, finally you could add one arm towel or rope chins, finger tip or chins or on a couple of digits I suppose.
I think you're right there are less guys who can do 1 arm chins than a 2x BW bench...but you could argue would that be the case if every Monday was 'chin day' & not 'bench day' & people asked "How much do you chin?" instead of how much you bench?
I can get virtually any non-obese young female with no shoulder issues chinning within 6 weeks. I've never been asked to actually to get someone to a one arm chin (I'm always better at planning for other people than for myself for some reason).
If I had to create a plan it would be.
stage 1
chin-ups using ladders working daily well below max capacity, working up to several times a day (low reps, nowhere near failure)

stage 2

After a short cycle of stage 1 I'd move to a 'same but different' idea. So, some days I'd do ladders unweighted, others I'd use a towel in one hand & over time work down the towel (do both sides), third weighted chin-ups. I would vary grip (supinated, pronated & neutral) & would also use gymnastic rings for variety & they do seem to be better on the elbow than a straight bar

If a guy can do chins I wouldn't bother with pulldowns if one arm chins were the goal. As long as a person can get 3 chins you can work up for there, first off using ladders (1 rep, then 2 reps, then 3 reps, then 2 reps, then 1 reps - with short breaks as though someone else was doing the same reps after you).

That would be one method. If you had limited time you could modify, but even just doing a few reps at home daily can really help get numbers up, the secret is to keep it well below failure, a chin is more like a push-up than a bench press if you see what I mean, a person can do it very frequently without overtraining if they keep numbers low & frequency high.