The article cites no real evidence that stock price changes of a small group of companies had anything to do with where the CEO worked previously and the article’s suggested next read in the Economist article is “How McKinsey Lost It’s Edge” which goes against the very point you’re trying to make.
It opens with the statement “As it nears 100, is the world’s most illustrious consultancy past its prime?” and cited poor performance associated with its advisors. So your “citation” isn’t really helping the point you’re trying to make.