It's because the grandparent comment has misconceptions about nutrition, including that animal products are necessary for a healthful diet and how much protein is required for a healthful diet.
The first misconception is contradicted by extensive analysis by health organizations such as [Academy of Nutrition and Dietetics][1]. Healthful vegan diets are no less healthful than healthful non-vegan diets.
The second misconception is about how much protein is needed in a healthful diet. Many people wildly overestimate how much protein a person needs, especially when strength training. It is not difficult to get sufficient protein within a given calorie budget while on a vegan diet, even without using protein supplements. For instance, tofu and seitan have protein to calorie ratios similar to meat, with seitan often having a higher protein to calorie ratio than most meats.