That would make sense. I was thinking of starting up fishing again and catch junk fish just to feed the dog. I can kind of see the vet’s reason with the vitamins - but wild dogs and wolves seem to do just fine without them! But like you say - they are getting all of the organs - not just the muscle meat.
I read an account once of some lady who oversaw the feeding of the seals at a zoo.
She did her best with them, carefully preparing, cleaning, and filleting the fish, and yet the seals began to show signs of nutritional deficiencies.
Turns out, seals in the wild eat the fish whole and she was actually robbing them of much needed nutrients.
She started feeding them the whole fish and they bounced right back.