However, the problem isn't meat eating; it's the quantity and fat content of meat that is eaten. Eating moderate amounts of beef, chicken, etc, is associated with longer life and healthier hearts, but eating too much is associated with heart disease.
He also doesn't mention that another thing the groups all have in common is being alive at the same time - if treatment for symptoms improved, all ages would benefit, e.g. with the creation of the National Health Service (NHS) in Britain, starting with legislation in 1948 and presumably taking a few years to ramp up to nationwide usefulness.
I also read some more of the "articles" on that site you linked to and they were absurd on their face. Here is one example. They are attempting to refute the claim that animal farming is an efficient use of land.
"Much of the land used for animal farming, cannot be used for arable farming. With a rapidly expanding world population, a large proportion of whom are already starving, how can taking this land out of production help?"
This completely ignores the fact that large percentages of our corn and soy farming is used to feed animals. Ignoring this single data point either makes them ignorant or dishonest.
I'd go google up the data about heart disease declining in Japan and continental Europe in the post-war era, but I get the impression you're blind to facts.
Regarding land, if we used the land used to grow animal feed to grow plants for human consumption we'd get a lot more food. This is clearly more efficient, yet the page you linked to tries to claim the opposite by ignoring facts and building logic on that false basis.
Animal feed is only grown on this land because it is subsidized. Growing plants on it is economically inefficient. If agriculture in the US were unsubsidized, nobody would try to grow plants on this land. It would be grasslands. And the only human use for the land would be to graze animals on it.
I agree with you about the current absurdity of our system.
Dairy cows and egg laying chickens and farmed catfish are much more efficient at turning vegetable matter into complete proteins than humans. If you accept the premise that humans need almost a gram of complete protein per kg of body mass per day, animal production is more efficient and quite necessary.