I've had many horses with great back muscles and I've never done a darn thing other than ride them. So I don't think you have to do ground work to get a good back. I really think it's a product of overall health and weight and the conformation of the horse.
The only horse I've ever had that has a less-than-stellar topline is my Foxtrotter. And sometimes I think I am just expecting her to be broad-backed like a QH but that's not her natural conformation. She was also a broodmare and had 10 foals before I got her, so that might have a little to do with it. I know she has a large belly even when she is in fit condition.
So I don't know. All my guys have had good strong back muscles with just good feeding and riding. I don't think you have to be doing ground poles and such. It depends on the horse's natural conformation as well. Some horse will never be built broadly.