Suppose you become knowledgeable about some area. You find there is an investment opportunity, so you invest. You tell your friends about it. Should they discount your knowledge because you've acted on it?

I happen to disagree with the article, but I didn't dismiss it just because it was written by someone with an interest.

Quite a lot of opinion about things in the world is written by informed people who have an interest in one side or another. You can read critically and decide which parts make sense to you and which do not.

just read it and see if it makes sense, if you are not confident to form an opinion, there is always the option not to.

Because it explains how equities markets work, mostly correctly, and this is background knowledge you need if you are buying or selling equities or forming opinions about capitalism or things like "clear conflict[s] of interest".

Clear conflicts of interest are usually fine. It’s the unclear or hidden ones that are sometimes problematic. I mean judges trust lawyers to argue cases on behalf of their clients with a declared conflict of interest to opposing counsel.