Why wouldn't nutrition scientists studying nutrition come to a similar conclusion about how young, murky, and complicated nutrition is? Shouldn't they on average know this better than anyone and only make very careful and strongly supported recommendations?

They have very strong incentives (ie earning money and building a career and having patients) to pretend to be certain. People don't want to pay for honest but vague guesses.

If you can't trust nutrition scientists to judge the literature properly, why should you trust scientists outside of the field or layman attempting to dive into the field would be better?

You shouldn't really trust scientists outside the field to talk about the entire field of nutrition but insofar as experts in older and more reliable fields like chemistry or biology disagree with specific nutritional claims you should probably agree with the actual scientists.