Because the parent used their anecdote of gluten intolerance to explain why caring about the gut microbiome matters. But traditional gluten intolerances are not related to the gut microbiome.

We also don't really understand why things like a FODMAP diet work. It's not that feeding your gut bacteria is bad, it's actually pretty good. But for some people it's bad, and they get symptoms they attribute to gluten intolerance.

Legumes, onions, whole grain etc that are high FODMAP are good for you. Fiber is good for you, it lowers your risk of metabolic diseases and helps your digestion. But, for some people, it's bad for their digestion. That's weird.

So all that is to say that, while gut bacteria matters, it varies person to person and we can't definitely say what food is good for the microbiome and what isn't.