The implication is there must be so many people trading as Turtles that there is no longer any opportunity to trade it profitably. Not only would they need to cease buy and hold approaches based on fundamentals they would need to start trading as Turtles. People would need to trade short as often as they trade long. The majority of people in the market do not trade short or are scared of trading short. The mutual fund industry and its mammoth size of combined assets would need to shrink dramatically and change their investing styles.
Most mutual funds are nothing more than buy and hold index buyers.
For Turtle trend following to lose effectiveness trends would need to end. Markets would need to go sideways forever. Most traders are afraid to trade outside of their knowledge area. Since most people only follow fundamental analysis, which requires you to be an expert in a stock or commodity, they lose out on huge opportunities. Turtles don't care if they are trading on a Paris exchange or a New York exchange.