> The self-reported dietary data suggested, unexpectedly, that the number of unique plant species that a subject consumes is associated with microbial diversity, rather than self-reported categories such as "vegan" or "omnivore"
Its a naturally occurring feedback loop. And if you get sick of split peas every night, you can seek varied sources of polyphenolic rich foods in surprisingly varied places: pomegranate seeds, cocoa powder, etc. Stay healthy :)
Additionally, since bacteria strains tend to be specific to their environment, i.e. the plant that they're living on (which the plant itself thrives better in particular environments), it makes sense that a diverse diet would lead to a more diverse gut biom.