In addition: isn't the gut microbiome downstream from diet? If all they are measuring is what people eat, we already know that!
Research confirms a wider variety of gut contents provides better health... "By increasing the variety of your diet, you can improve your gut health and overall well being" (and) "A diet rich in these foods helps maintain microbiome diversity and can reduce the risk of several diseases, including type 2 diabetes and cardiovascular disease"
https://groundcover.grdc.com.au/innovation/industry-insights...
... developed a ranking of species most favourably and unfavourably associated with human health markers, called the ‘ZOE Microbiome Health Ranking 2025’. This system showed strong and reproducible associations between the ranking of microbial species and both body mass index and host disease conditions on more than 7,800 additional public samples. ....