I’m not seeing how that indicates horses “never adjust” to shows, just that exercise while at horse shows is more stressful than exercise at home. That makes sense, since riding at a show comes with added stressors from the environment, activity, rider nerves, etc. I think a more compelling piece from that article was the reference to a study that showed increasing physiological signs of stress on each day of a three-day show, but the test subjects were noted as inexperienced young horses so it’s still not great science.
And FWIW I do agree that shows are stressful for horses even outside of their actual “work,” especially longer shows. I don’t need scientific proof to accept that as reality. Some horses are more sensitive to it than others, but we know all horses are pretty good at masking stress so it can be hard to say for sure how they’re feeling. Even the ones that seem to adjust are still dealing with higher than normal levels of stress to some degree, and the effects of that can wear on them over time. There are a lot of things we ask our horses to do that are “stressful” and not all stress is the end of the world, but personally I think the multi-week shows are up there for sure.