Lindeberg is a nutrition researcher (conducts studies, co-authors papers) coming from a medical background, which makes him just as much an expert as a nutrition researcher coming from a biochemistry background.

Why wouldn't nutrition scientists studying nutrition come to a similar conclusion about how young, murky, and complicated nutrition is and only make very conservative, very strongly supported recommendations?

We can measure how much a field has progressed by its predictive power, and nutrition is already making concrete predictions with high confidence. Not a lot, not with the confidence of, say, Newtonian mechanics but, given how very much literature there is and how very complicated things are, the level of consensus across researchers who are coming at the problem from disparate-but-legitimate approaches (e.g. biochemical, evolutionary) is sufficiently impressive that I do trust them to judge the literature properly. Humans are biased, so it's unsurprising that we don't yet have a consensus as broad as, say, existence of the golgi apparatus, but the world looks exactly as we'd expect it if nutrition scientists were doing good work in a complicated field.

To summarize: Lindeberg, like Campbell, is an experienced nutrition researcher with impressive and relevant credentials. Nutrition is a young and complex field, so there's no broad consensus about everything—although there is broad consensus about some things—but nutrition scientists are doing a decent enough job of figuring things out that I trust them to judge the literature properly.