I wouldn't go so far as to say unscientific, because as you say we are only partially adapted to it (e.g. lactose intolerance), and grains are an extremely easy source of calories which is problematic in a context of the obesity crisis and problematic when it's only simple carbohydrates being consumed by poor people such as potatoes or white rice (vitamin deficiencies). I don't expect epistemically sound and measured statements from a community around a diet that's become a social media fad, anyways.

I understand we can slip into the naturalistic fallacy easily, but at the same time I appreciate the line of reasoning as a first approximation because of how complicated the human body and nutrition is, and how messed up our public health situation is thanks to the modern diet. Even though I acknowledge this situation is attributable to processed foods and sugars, and not to whole grain consumption which I think is fine/good for most people, it is still the delta between modernity and how our bodies evolved that is the culprit.

Let's put it this way. The Paleo community is probably totally wrong about whole grains, but right about a lot of things.