The article isn't referencing index investing as a strategy; it's talking about a strategy in which, for a while, you could make money by buying specific stocks when they met the criteria to be newly added to an index. That appears to no longer work, because markets have become more efficient and the value is already priced in.
People thought they could beat the market by timing buys of stocks that were likely to become part of indexes.
If there was any alpha in this practice, it’s been sucked out of the market at this point. The anticipation of this has already been priced in, so there’s no remaining advantage in trying this strategy.
