I think it's coming from this source (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5872783/) which more or less just generally claims that "Western Diets" are disruptive to gut microbomes.

Annoyingly, it doesn't really pin down what exactly it characterizes as "the western diet". It does include extracted oils and refined simple sugars (glucose and friends). But where exactly stuff like pasta/rice/flour sit is less clear in their description.