This is just a correlation, not a causation.
The real factor influencing life expectancy is likely going to be wealth. Poor countries have little/expensive meat and a low life expectancy. Rich countries have much/cheap meat and a medium life expectancy. And wealthy people in rich countries will have the highest life expectancy, even though they're more likely to eat a vegan diet compared to the poorer people in those countries.
As a result, if you look at it globally, on average, people with a high life expectation are eating tons of meat. But if you look at just rich countries, people with a high life expectancy are eating less meat.
And if you're thinking, the green people are faking studies, consider for a moment, if we would really deem life expectancy worthwhile, with the planet getting gradually less worth living on from here on out.