Of course, nothing in life can always be 100% gaurenteed, but wouldn't you say a breeder that breeds two dogs that are healthy and without HD would be more likely to produce healthy pups without HD than some BYB that is breeding dogs that have not been health tested?
Of course
But, I have seen first hand well thought out litters of certified bitch and dam with excellent hip scores throughout the lines produce dysplasic pups. One friend with health tested Bernese Mountain dogs both excellent in all health tests produced a completely dysplastic litter, she had to destroy all of them.
My Newf which is a champion in Canada, States and Bermuda has Elbow Dysplasia.
A certified dog with good scores at 2 can still develop HD later in life as well. We have one Newf at the kennel crippled with HD, he had perfect scores at 2 years of age. Sometimes lifestyle, slippery flooring, diet, weight etc can cause a dog to develop HD. Also with purebred dogs the gene pools never get any bigger, they can't....a breeder sometimes has no choice but to inbreed. I consider inbreeding to be any family member...most breeders consider inbreeding to be only father to daughter, brother to sister etc. They consider breeding cousins etc. to be linebreeding. So, this breeding can promote these health problems as well as unhealthy lifestyles.
You can get all the supplements I recommended at any store that sells vitamins.