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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.

You can look up SAD, the standard American diet for a reference.

They displace food containing more fiber and by doing that change gut flora to that which does not produce IPA.

Fruit is not a refined carb...

I assumed that simple sugars count as "refined carbs" and simple sugars are found in lots of stuff including fruits. Btw, my hope is that this very restrictive diet actually isn't necessary for this metabolite.

By definition they aren't refined. You would have to remove the fiber to make those sugars refined as a first step. Fruit isn't pure simple sugars and it certainly doesn't act that way in the gut when buffered with fiber.