I think the part of the long back being a "blessing" is that long backedx horses are often more comfy for the rider, and less of a challenge to fit to a saddle.
I think your horse has a long back, but more than that, I think his hind legs are camped out a bit. And the angle of hip is on the flat side. If you position his rear legs so that the canon is perfectly vertical, it will be behind the point of the buttocks, thus the camped out. And when he bring them under himself, he appears sickle hocked. This is really pretty minor.
But you can see that undersaddle, he stands a tiny bit camped out in back and there's a noticeable drop in his back. Since I am such a chunk that I weight half again what you do, I would not want to subject this horse to my weight on a daily basis.