> I was in the academia for some years and didn't find in my field any expert that was incapable of distinguishing well proven Vs opinion.

They are capable of it, but they do not always bother to do it when talking to a non-academic audience.

One example that comes to mind is how advice on diet has changed over the years because advice was given (e.g. avoid dietary cholesterol) that was not well proven. Another was the introduction to a fascinating book I read a few years ago on the complexities of evolution (e.g. genetic changes with multiple effects, gene transfer, etc.). In the introduction the authors explained that colleagues had pressured them not to publish because it differed from the simple model of evolution taught in schools so would encourage creationists.

Economists are often biased towards their own political opinions.

To play devil's advocate, I think there is some merit to their point. A common criticism against modern academia is that so much of the low hanging fruit has been addressed, that to be an expert working on the actual frontier means your "leaf node" of focus/progress is so so specific, that it's not at all uncommon to be somewhat blinded to the broader consensus/lower resolution version of the greater tree of knowledge.

The experts of the kind to which you're presumably referring, who are much more tapped in to the whole state of a broad branch of knowledge, often end up being more "science communicators" than people on the frontier of research. The thing is, though, that these "science communicators" often end up being (or start off being) more akin to a papers/Wikipedia jockey than a credentialed academic actively working in the field. So in that sense, someone who "knows a lot about dinosaurs and can effectively write about them as the broader field currently perceives them" need not necessarily be a credentialed expert.