And you would expect people who eat a vegetarian diet to be more healthy on average. They're likely more affluent, educated and aren't drinking, smoking or eating junk food as much as the general population.
I don't have access to the article, so I'm not sure what they define as a plant-based diet, but Harvard defines them as a diet focused mostly on plants, not one that omits meat (vegetarian) and/or animal products completely (Vegan).
I wouldn't argue a diet based around plants is more healthy than one that is not, but I wouldn't generalize and say vegetarian (read: no meat) is more healthy than one that does involve meat.
> How is "marginally more healthy _at_ best" defined?
Just comparing the nutritional values between 4oz of beef vs 4oz of Beyond Meat.
Studies done in the USA can have very disparate conclusions because questionnaire studies are terrible w.r.t. diet "in your study, "The ARIC study did not assess whether participants were following a plant‐based diet.", the average person eats insanely unhealthy (> 70% overweight) and the average american's idea of meat is basically fried bread with condiment levels of meat. It's further confounded because vegetarians are usually higher SES or conscientious about what they're eating in the first place.
So you would expect to find studies disputing this and also studies that find the health effects of vegetarian diets are not unique. And you do:
(2012) Vegetarian diets, low-meat diets and health: a review. Public Health Nutr 15: 2287–2294. https://www.ncbi.nlm.nih.gov/pubmed/22717188
> CONCLUSIONS: The health benefits of vegetarian diets are not unique. Prudent plant-based dietary patterns which also allow small intakes of red meat, fish and dairy products have demonstrated significant improvements in health status as well. At this time an optimal dietary intake for health status is unknown. Plant-based diets contain a host of food and nutrients known to have independent health benefits. While vegetarian diets have not shown any adverse effects on health, restrictive and monotonous vegetarian diets may result in nutrient deficiencies with deleterious effects on health. For this reason, appropriate advice is important to ensure a vegetarian diet is nutritionally adequate especially for vulnerable groups.
2017 study shows "no evidence that following a vegetarian diet, semi-vegetarian diet or a pesco-vegetarian diet has an independent protective effect on all-cause mortality": https://www.ncbi.nlm.nih.gov/pubmed/28040519/
An Austrian study suggests vegetarians are less healthy and have a lower quality of life, and higher rates of mental illness: https://journals.plos.org/plosone/article/file?id=10.1371/jo...
In Australia, which I think has one of the highest meat intakes in the world, "We found no evidence that following a vegetarian diet, semi-vegetarian diet or a pesco-vegetarian diet has an independent protective effect on all-cause mortality."
Vegetarian food is healthier, and reduces all cause mortality. Here's one of many studies that say so:
https://www.ncbi.nlm.nih.gov/pubmed/31387433