I don't doubt that there are strategies and traders who can add skill-based alpha to broad market returns, but the catch is that it is essentially impossible to attribute performance to skill or luck. You may say consistently outperforming the market for 5, 10 or even 20 years is the mark of a great investor, but given the number of funds and players in the space, simple statistics would conclude such outcomes should routinely occur. You see it again and again where some manager has a remarkable run, raises tons of money, and proceeds to encounter mediocre/terrible performance. Some former "genius", do-no-wrong managers have actually lost way more client money than they've ever made because they attract tons of money after a lucky run and then proceed to tank.
The silliest part of the hedge fund industry is seeing all these funds and media retro-fit all kinds of rosy superlatives and sophisticated explanations onto a good year or two run, without ever acknowledging the likely role of luck and randomness.
Warren Buffets essay "The Superinvestors of Grahm-and-Doddsville" perfectly captures so much of this dynamic in a fun hypothetical exercise: http://www.tilsonfunds.com/superinvestors.html