Sanchin was a "body-building" kata. By that I don't mean Arnold type bodybuilding. What I mean is that Sanchin builds a "karate body" to strike with speed/power and the ability to take blows. It developed what the Okinawans called "chinkuchi"
Chinkuchi is only partially involved in sanchin kata, and less so in other katas. (see next quote for explanation.)
Part of the idea of chinkuchi as I understand it is that the muscles come into alignment at the moment of impact
This is a better statement of chinkuchi, the definitive phrase being "at the moment of
impact." It is the total delivery of power combining muscle, speed, biomechanics, alignment and breath at that moment. Sanchin is mainly devoted to developing a rooted stance to the ground and overall stability while force is transmitted horizontally to the opponent.
The reason chinkuchi is not a big part of sanchin is that there is no speed (except in some of the Uechi-ryu moves) and the impact is not the main concern, rather what's happening on the way to impact.
Uechi ryu has the remnants of Chinese iron shirt training
There is some of this element present in sanchin. Iron shirt is not much related to chinkuchi.
I’m not trying to critisis, but rather understand. Goju- and Uechi Ryu have a long and noble history but for me, this extended training time spent in ‘body toughening‘ to produce ‘organic armour’ doesn‘t make sense
The dynamic tension in sanchin which provides some "iron shirt armor"
is different from "body toughening." I think you are referring
koteki tai, usually known as body conditioning by arms, fists and legs being repetitively impacted by a hard object (such as stick, makiwara, tree, or partner's strikes) to strengthen the bones and muscle.
As hoshin1600 said in his quote, Okinawan styles have but a remnant of this armor in their training." More common is koteki tai conditioning (while "kote" refers just to the arms, it's often used as a general term).
While all of the terms I've been throwing around can sometimes intersect in varying degrees causing much confusion, they are separate concepts. I hope I reduced this confusion and not increased it.