i am not sure about auxins helping the healing aspect of things..perhaps??? but if you cut your cactus, or plant for that matter, the auxins are going to concentrate elsewhere to feed a growing point....this could be a pup or a scion. i would *guess* that with species like T. pachanoi, which often pup from the base, that if cut and left for a period of time the auxins would be concentrated at the base of the plant.
but you are right in that new growth, already being hormone filled, will grow much faster than older growth. that said even 6" thick 4 year old woody columnar tissue is graftable.....but success is low and growth even lower. i think it has to do with the flexibility of attachability of new soft tissue, rather than growth hormones (excuse the english lol). even then, old growth can still be somewhat soft and totally graftable. "this years growth" is just a quick guidline, but is by no means the rule.