A genetic test is not a clinical diagnosis, but it might help your vet make a diagnosis.
Remember, Embark tests for genetic variants that associate with increased disease risk. Only your veterinarian can determine whether your dog actually has a clinical disease, which involves evaluation of your dog's presentation, history, clinical diagnostics such as bloodwork or radiographs, and yes, sometimes a genetic test. Your vet is trained to tie all of this information together to paint a clinical picture of your dog, which ultimately will inform their diagnosis.
If my dog is at increased risk for a genetic health condition, what's the likelihood that my dog develops a disease? It really depends on the condition. Some mutations are known to have incomplete penetrance, meaning not all dogs at increased risk will go on to be diagnosed with the disease, or variable expressivity, meaning that a dog's clinical signs might fall somewhere along a spectrum of disease severity. But as we've noted above, visiting your dog's veterinarian is the first step into determining whether your dog might be showing signs of disease or not.