I appreciate your input, but this level of categorical statement seems to fly in the face of historic and current diets which are vegan or vegan-like and have not relied on supplements, or at least not the type we are talking about here.

At any rate I'd review other dietitians/scientists as well; while it's been a long time since I read the literature on this (when a family member went vegan) your quote certainly wasn't consensus view at that time.

Absolutely agree if you were to eat such a restrictive diet, you have to pay attention to vitamin sources, or you can get sick. B12 particularly problematic because you don't need much at all and can go months/years in a deficit situation before showing any symptoms, which can make it hard to pin down.

(To be clear, when said family member did do this for a while, I suggested supplementing but the nutritional science types I was reading weren't nearly as categorical as your above quote)