I have heard the same from many physical trainers. And makes good sense since machines are easier on joints.
I'd think machines are easier on the joints, for one thing because you don't have to correct for random movement of free weights.
In my case (I'm 68!), I've had joint issues sometimes better sometimes worse for years. The term would have to be "chronic" but it does vary a lot, which gives me hope, of course. I really should try some of the stretches and special exercises. I was shown a few but haven't been doing them. Hey, when the pain goes away I forget about it. Pain's funny that way, and like my nephew said "it's always something," or maybe I said that. Thing is, if you aren't in pain you push it until you are, well I'm like that. If I had no pain issues I'd continue to pile on the sets and weight. I'd do that until I had issues and I'd have to back off. To avoid issues, you have to be basically fit and you have to train smart.
Lee Haney, 8 consecutive Mr. Olympias, used to say that he learned a lot from the older guys about protecting his joints, how crucial that is. Guys like Albert Beckles, who at 59 had a massive physique (a magazine I saw). Haney also said that body building was 85% about nutrition. I don't think I've gotten my head around that one.
I think one of the techniques is to not go too heavy too much. In my case, my chief strategy (like I said I think I should do more stretching and special exercises) is to go light and do a lot of reps. The only exercise I'm doing right now that I don't do at least 15 reps (I usually do 30 or more!), is my chinups. I'm doing 10 full clean chinups now, I max it out. I do a pretty maxed out set of curls with a curl bar at the end of my workout, 15 reps. Oh, and I'm taking Glucosamine, not sure it helps but WTH.