You are right. You can't trust what most vets say and you can't trust what some people on the internet say. Ellie was having some liver problems and I asked the vet if we should go to low protein. She said it wouldn't hurt and said we could use Science Diet L/D. I said no. She asked why and told me she doesn't use Science Diet much either.
She said that she learned very little about nutrition in vet school when we told that we were homecooking. She siad to look online. Unless your vet has studied independently, they got about one or two nutrition classes in vet school. They probably only learned about essential nutrients (according to AAFCO of course).