I don't think the (strong) version of the efficient-market hypothesis is defensible but Warren Buffet's track record is not proof that it is wrong - in a random market, given a large number of investors, some will do well purely by chance.
Not necessarily refuting what you're saying, but here's an interesting take on "purely by chance" from Buffett, by analogy of coin flipping and getting a run of heads: http://www.tilsonfunds.com/superinvestors.html
That's a great article, thanks. I actually had the coin example in mind when I wrote the comment, but was too busy to write out the full argument!