It's unclear to me whether this is actually a causative factor, or more correlative. Is it that people who eat these diets are generally more health-conscious in general, and that's why they're healthier? You're already selecting for a population who is more thoughtful about they're eating than your "standard" burger-and-fries eater.
Mind you this is an umbrella review, analyzing the results of nearly 50 studies published in the last 20 years. As per the study[1] methodology: "PubMed and Scopus were searched for reviews, systematic reviews, and meta-analyses published from 1st January 2000 to 31st June 2023, written in English and involving human subjects of all ages"
It's a comprehensive (more or less) review of the relevant literature than an one-off study.
They acknowledge some limitations: "A key limitation is represented by the high heterogeneity of the study population in terms of sample size, demography, geographical origin, dietary patterns, and other lifestyle confounders"
