Ian, that is a great question. I learned how to write in academia, and I received the same advice you did. However, some of the best writers I read ignored that advice. I liked them precisely because they seemed more honest and humble.
I think the trick is to know your subject well enough to understand when you’re making a stretch, intellectually speaking, and when you’re just saying something that is generally regarded as true. It’s a fine line, which is why writing non-fiction is difficult to do.