Because there is no DNA test the only way for a breeder to really have a good idea of how likely it is to appear in their breeding program is to have vertical pedigree information. This means getting data about the siblings of the dogs in a dog's pedigree. Though Walker and Robyn may not have patella issues, have their siblings been tested? What about their grandparents and their siblings? Even then a breeder has to make an assessment of risk and then decide if it is a risk they can live with.
With some diseases, people have not done enough testing to have a really good idea what to expect. Also, if other people are not willing to make their test results public then it is not available to use when making the decision.
Both Zip and Walker have some dogs in common in their pedigrees, Avongara Gangura and Zamee. Zip and Robyn also have a dog in common in their pedigrees, Serengeti Cool Jazz of Woz. Also, Robyn's grandsire has sired a dog with a patella issue listed in the OFA database.
That doesn't mean Denise was irresponsible or could have predicted that there would have been a problem, just saying that even though both Walker and Zip were bought from other breeders, Denise has a certain family of dogs she works with. Denise did the responsible thing, she had the puppies examined before she placed them and has been very up front with all her buyers. I am sure she will also keep in touch with them to see how the pups progress.