People need to take the efficient market hypothesis seriously when using stock prices for things like this. The announcement of a new CEO (or more realisticall

People need to take the efficient market hypothesis seriously when using stock prices for things like this. The announcement of a new CEO (or more realistically, the rumors preceding the announcement) already given the market a chance to adjust based on their expectations of the CEO. So what the study really shows is that highly paid CEOs tend to perform less well than the market would expect. This might be because they perform worse. It might also be because they perform better, but not by as large a margin as the market expected.