There are a lot of strong claims that the paper could, but does not make. It never says that Congressional leaders outperform index funds. It just says very specifically that leaders outperform other members of congress. The paper also does not include any clear charts on the actual returns the congressional leaders were getting.

Having done some reading on this myself I don't think it's the case that Congress as a whole outperforms SPY [1]

[1] https://www.sciencedirect.com/science/article/abs/pii/S00472...