I had a vet tell me this and in OPs defense, I have seen this online before. It is based on dominance theory I believe. The vet said, make sure the old dog eats first, gets your attention first, goes through the door first so the puppy will know his place in the pack. I have heard too that this can be a disaster. In my case, my puppy was always going to be the more dominant dog. I think it is much better to think of yourself as leader, you set the rules and the puppy pays attention to you. I have always fed my dogs together, when I had multiple dogs. They have to wait for a release and they do not get to clean the other guys bowl or approach the other dog while eating. My rules.
I would feed them at the same time but a distance from each other. I would put a long line on the puppy and if he tries to approach the other while eating it is a firm no and a redirect to a toy or treat.
Dutch Shepherd - Ptygo (Tee-Go) de las Flores
Rescue GSD - Freyja (Husband's Dog)