Nara was diagnosed with moderate HD in both hips at a little over a year old. There was already some arthritic change even that young. We were told to expect major surgery or she'd be crippled by midlife, and that was from a very experienced ortho vet.
Well, good thing dogs don't read x-rays or diagnostic reports because at 11 yo she's still going strong. Just in the past year we're starting to see some symptoms, but they are very mild..... doesn't run as fast as she used to and bunny hops a bit, doesn't have the stamina she used to, takes her a bit longer to go up the stairs than it used to, a bit more difficult for her to get on the bed and couch. But really, I'd expect those things in any 11 yo dog, HD or not.
We put her on supplements as soon as we found out (glucosamine, chondritin, MSM, Ester C, fish oil/vit E) and of course have always kept her at a good weight and given her appropriate exercise to keep up muscle tone (which helps take some strain off the hips) and in playing avoid things like bouncing kongs that can lead to a drivey dog putting all sorts of incredible strain on the joints. Haven't x-rayed her since then and probably don't want to know what those x-rays would look like now a decade later, but it doesn't matter as she herself is fine.
So while I know it's very concerning news to hear that from a vet, based on my experience I'd say it's safe to assume that as Elsa isn't showing any signs now the long term prognosis is very good regardless of what the x-rays show. Good that you know this and can start a management program to stave off symptoms as long as possible, but overall I'd say the outlook is pretty bright.