First...get off the internet.
Second...go to a working dog club and see.
Third...drop the "slanted back" thing. It gets so tiring. You should know that a straight back/slanted back isn't always a way to tell apart a working dog from a show dog. There are much bigger signs that should be focused on rather than the slant of the back.
A GSD back shouldn't be parallel to the ground anyways. There should be slope to it because of the angulation in the back end. Now, how extreme that slope is, that's up to the breeder and who ever is interpreting the standard. A slanted back has NOTHING to do with the quality of the hip joint. Just look at your dog and imagine how you would lower the back end...it would have nothing to do with the hip joint. HD is how the ball fits into the socket...the angle at which that happens does not affect that "fit" what so ever.
All large breeds have HD. It's genetics, and we don't completely understand it quite yet. I have a friend who went to a "reputable breeder" and got a dog out of a dam and sire that were both rated normal/excellent. His dog ended up with HD. I went to a "BYB" and ended up with a dog that has a normal/normal rating. Do BYB dogs have a higher instance of HD? Who knows? People buying from BYB aren't generally getting their dog's hips rated...so the statistics are actually mostly based off of show dogs and trialing dogs...which generally come from "reputable breeders."
Blaming genetic issues with the breed on BYB is extremely overreaching. Plenty of good breeders do things that can easily be questioned. Not many breeders out there have had 10+ litters without one instance of a genetic disease or issue cropping up. It just depends on what they do after they see that.
The reason you see working line people have problems with show lines is that they don't have the temperament to work. It has very little to do with their look and more to do with the fact that the GSD is a working breed, and there are a lot of show lines that just can't work.