Most investors, both institutional and individual, will find that the best way to own common stocks is through an index fund that charges minimal fees. Those following this path are sure to beat the net results (after fees and expense) delivered by the great majority of investment professionals. Seriously, costs matter.
True, he doesn't say you can't beat the market. But he does say, then and now, that you probably shouldn't bother trying.
Plus, say through in-depth research and know-how, you do manage to beat the market. At best, you've created a part-time job for yourself. Unless you're managing other people's money, or have a 7-figure+ portfolio, I expect the time taken to do the due diligence necessary to consistently beat the market - if indeed such a thing is possible - would end up paying a lower hourly rate than you could earn doing other, more reliable, less zero-sum things.