while i wouldn't call them the same discipline by any means, but a lot of the goals are the same. Both have the goal of ultimate control over the horse with minimal aids. You shouldn't see what you are asking from the horse in both reining and dressage, its all done by the seat and minute leg aids.
they also do a lot of similar moves, they might not be exactly the same, but you can tell what they may transfer to, like the changes of lead, spins (pirouettes!), even moving sideways.
I consider reining western dressage, dressage means training correct? its a test of how well you've trained the horse to respond to your aids without you having to be loud about them. Reining is testing the same thing.