3 ideas
Market efficiency is graded by which information set prices already reflect. Weak form covers past prices and returns, so technical analysis cannot earn excess returns. Semi-strong form covers all public information, such as earnings, announcements and splits, so fundamental analysis of public data cannot either; strong form covers all information, including private, so even insiders and professional managers cannot.
Market efficiency can never be tested alone, because 'excess return' only means something relative to a model of what returns should be. Any anomaly is therefore evidence either that the market is inefficient or that the asset pricing model is wrong, and the data cannot say which. This makes efficiency a lens for organizing evidence rather than a claim that can be cleanly falsified.
Cumulative abnormal returns rose steadily before the split and showed no reliable drift afterward, once dividend news was accounted for. The market had already priced the information that splits signal, so trading on the public announcement earned nothing. The study became the template for event studies testing semi-strong efficiency.