Well, he would say that wouldn't he?
Thornit has been used for many years by thousands of dog owners to great effect. I have used it on Coco for 10 years and recommended it to numerous people who have found it worked when nothing else had. It often does a better job than the stuff you get from the vet, and for a fraction of the price
I once took it in to show my vet who had also never heard of it and he said that it contained some of the similar ingredients to the stuff they use but was much cheaper. He didn't have a problem with it if used correctly.
I have learned over the years that vets do not know everything and so I do my research and then make my own decision. Many vets are against raw feeding despite plenty of evidence to the contrary - nothing to do with them trying to sell you expensive, dry dog food then
I have also used essential oils, homeopathic remedies etc and never had a problem - I am happy with the stuff I use and, if I think the vet might disapprove then I just don't tell them. In fact my vet is really good and even if they do not agree, they do know that I have done my homework and have very good reasons for using the things I do.
If you want to use Thornit and you find it works, then it is entirely up to you what you use. The vet is there to advise, not to dictate how you care for your dog.