And you fail to take into account the healthy user bias. Plenty of people become vegan because it's advertised as healthier, which means it filters people that don't want to be healthy more than some other diets. Also, vegans are very aware that they diet may lack X or Y so they pay more attention to nutrition. For example, lack of B12 was a meme against vegans at some point, which means now most of them are aware of B12. I can't say the same for the rest of the population.