Okay, as far as I can tell, that's just straight up lying.
Vitamins and minerals that vegetarians and vegans tend to lack:
Iron, B12, creatine, carnosine, Docosahexaenoic Acid (DHA)
Source:
http://authoritynutrition.com/top-5-reasons-why-vegan-diets-are-a-terrible-idea/
(Iron isn't specifically listed that I saw, although I can cite other sources naming it as a common deficiency)
Also, according to that article:
...but ...but if you eat just exactly right, you can avoid this!
Maybe so, but vegans have been shown to tend to be nearly uniformly deficient in these areas. I'd say creatine and/or testosterone shortages are the most likely causes for the effect noted, although I don't presume to know. Nutrition isn't my area of expertise by a long shot... so I have to look somewhere other than my own mind for that information.
It's a little bizarre to claim that having a diet different than everyone else will not produce a biology that is in some way slightly different than everybody else's. Whether you can say those differences are "unhealthy" or not can be a matter of opinion, I suppose, and I'm not even sure that the date when you die is even the only meaningful measure of that. In any case, I'd say that it stands to reason that our bodies (and by extension, our minds) might tend to notice those differences when presented with a new diet... I don't think that's an unreasonable assumption at all.