I haven't really followed her story that much but did she actually produce test results or did she just want a way out? She lives a fast, stressful life and (not judging, talking strictly from a health perspective) has done a shit load of drugs and partying; at the height of my work-life balance struggle and substance use, I also could've said, "Oh no, my brain sure is foggy!"
This premise starts to fall apart when you ask yourself that question with regards to the pool of vegans to draw on in the first place. The lack of vegans in any place can't necessarily be attributed to them being vegans, especially considering non-performance issues people don't go/stay vegan. There are vegan Olympians, military personnel, firefighters, and the like though.
Wait, so his argument against veganism is that if enough people don't take the plate of meat there won't be any meat to replace it, but you shouldn't decline to take it because it doesn't matter to all the dead animals already on plates, even though it potentially could make a difference to the billions of animals who will end up on them in the future? Oooooookay.