I'm not deeply familiar with all the things he tried, but from what I understand, he tried many different changes in nutrition and lifestyle. He also attempts to optimize his sleep schedule and exercise plan for his goal of slowing down/reversing aging.
Nutrition is a very big component of his plan/blueprint and excluding a very big chunk of the search space (animal based foods) doesn't give me as much confidence that he found an optimal or close to optimal plan (especially compared to the effort required) to achieve his goal.
If the vegan diet was one of the outcomes from his experiments (where the goal is to optimize for anti-aging), I'd be fine with it. But it's a restriction he put on himself based on a ethical/philosophical motivation.
I really wish he did his research without being restricted to a vegan diet. He mentioned in one of his videos that his choice for a vegan diet isn't motivated by health, so I can't help but think that part of these recommendations are just there to mitigate the effects of that specific diet. For example, an omnivorous diet might not need certain supplements.
Nutrition is a very big component of his plan/blueprint and excluding a very big chunk of the search space (animal based foods) doesn't give me as much confidence that he found an optimal or close to optimal plan (especially compared to the effort required) to achieve his goal.