You can't put a population against another one, there is allways much more differences between two populations that just if they eat meat or not.
Compare my for country, Sweden, with India. In Sweden we are mainly meat eaters. In India a rather big part of the population are vegetarians (50 % in the southern parts). If you compare life expectancy, we live much longer in sweden. But the reason for that is not the meat. So just comparing two populations and look at one ingredient in the food is not scientific.
You need to compare two groups of people living the same lifestyle except for meat as an food ingredient. Otherwise your results will be weak, cause to all other factors that can interfere.
If you make an comparison that is "fair", you will probably find no differences between vegeterians and meat eaters.
In sweden we have among the longest life expectancy in the world and if I would reason, like you do, I would say it's because of the meat. But the real reason is that the standard of living is high, with a good wellfare for everyone.
I'm sure you can find other differences than just meat in the fare betwen Mormons and Adventists that can acount for the differences. I would be much surprised if two different religous groups had the same lifestyle. I haven't, however, read the article, and would be pleased if somebody could give me the references. Thank you!