If you have read anything about Warren Buffett, you would know that he does not define risk like we have been taught in school, i.e volatility. He also only invests for the long run so having said that, an index fund makes perfect sense with that allocation.
I'm sorry to break this news to you, but not everyone believes or wants the academic version of portfolio allocation. Buffett has done well without it and most likely better than those bashing him here.