You cherrypicked that last sentence, and reiterated what the N-1 sentence said.

I think the point is that this study suggests a causal relationship (having leadership power enables one to make more money), which is stronger than just correlation (being in leadership positions correlated with making more money).

But added "annually"! That's a lot better!

Here’s the strongest steel man I can provide:

A person typically picks stocks based on some view of the future: what’s going to be valuable, how the world is going to change, etc. Presumably, a congressperson does the same when drafting legislation - try to move the world in the direction they think it should go. The underlying worldview and outlook is the shared variable, and congresspeople in leadership positions are more capable of moving the world the way they think it should go than anyone else. The increased stock performance is because someone in congressional leadership’s predictions for the future are more self-fulfilling than most.

(Again, that’s the best steel man I can offer, and if you made it this far without laughing, I commend you)

Just require their stock allocations to be made public and anyone can follow their example.

Aren't they required to make their stock trades public?

Positive assortative matching aka rotten eggs stick together