It’s my experience with the KS horses I’ve personally seen. We’ve all witnessed people say “my horse is sound and the set up is perfect” and they show a video of a lame horse or inappropriate set-up. People from the outside looking in might see things differently. Just because someone is not your personal trainer doesn’t mean they can’t see an unsound horse. Trainers have fiscal motive to work a borderline horse. I wouldn’t use them as a metric. YMMV as always.
Yes, many of KS horses are rideable. I fully agree and in most cases advocate for fitness work for these horses. I am only cautioning you not to conflate “rideable” with asymptomatic. Being mindful of the disease and how it affects the horse helps manage it better and sets the horse up for future success.