That's an oversimplification.Buffet said "diversification guarantees mediocrity".
Further - what is mediocrity? Serious question.
If mediocrity means market results without fees over the long term, ala a Vanguard S&P 500 Index fund (they created the product), then you have outperformed 95% of the actively managed funds - and a 95th percentile performance may be "mediocrity" for some, but for nearly every single small investor - that is a grand slam.
Buffett has a team of hundreds, who can do research, go to companies, put "boots on the ground". No small investor, myself included, can ever hope to match that level of analysis.
So stop trying.
Be very, very happy with mediocrity - start early, don't panic, stay the course, and that mediocrity will lead to you being a millionaire.
Simple.