I agree with you that when coming from a hunt seat background that it can take a good bit of work to stop leaning forward and sit UP and balanced. It’s an issue of muscle memory in my case. For me, stirrup length makes very little difference in this, though, and maybe I’m just odd in that way. What really affects me is the saddle’s balance. I find it very difficult to ride with a balanced seat in most close contact saddles because the balance point is forward, no matter how long my stirrups are.
The opposite is also true - a dressage or all purpose saddle with an “upright” balance puts me in the right spot, even if my stirrups are shorter than what some would like to see. (Please know that I don’t mean cross country length stirrups, just not what a lot of dressage riders prefer.) I am no ULR, nor will I ever be, but balance has always been easy for me, even if other things about riding aren’t. So maybe a shorter stirrup length is more of a “handicap” for some riders than it is for others, while things like rider saddle fit may play more of a role in setting them up for success. I know there are ULRs who can hop on just about any saddle and perform well, but I could never be one of those.