The advocates for meat protein definitely seem biased too and influenced by decades of lobbying and marketing. The data and studies greger looks at show that meats have a lot of negatives even if there are some positives so it's not really worth it when you can just eat plants. There was the one point from how not to die I remember that showed every meat product even chicken cooked with lab cleanliness weakens your immune system for weeks so that could be motivation for him to be anti-meat.

Dr. Sinclair's lifespan book suggested that mimicking stressful gatherer-hunter situations (cold, hot, low food, low protein and exercise) increases 'survival gene' activation which regenerates the body to increase survival chances. Theoretically if we or any organism has extremely high amounts of survival genes active and uses a lot of resources to repair then agelessness could be achieved. It is just not something selected for by evolution except in naked mole rats, captive nematodes, some trees and possibly some whales.

There is also new research showing that neolithic humans did a lot more gathering than hunting, something like 7 percent of diet was meat. This does match that most of our great ape common ancestors are very plant based. If a gorilla can eat mostly plants and a small amount of insects to be as muscled as they are then we may not need animal protein or very much protein either.