You'll never be Warren Buffett or Bill Gates if you do X because neither of them did. Which misses the point that it's incredibly unlikely that anyone would ever be as successful as either. Most people probably won't be a Warren Buffett or Bill gates and optimizing for that statistically unlikely outcome could be suboptimal for the vast majority of people.

It also makes the mistake of assuming it's an "if and only if" argument. One becomes a Warren Buffett or Bill Gates, if and only if, they don't do X. But there's no proof given for X preventing becoming a Warren Buffett or Bill Gates. The lack of X in some small sample isn't enough to prove this implicit part of their argument.

Their argument fails on multiple levels. Maybe Masterminds aren't great for everyone. But that's orthogonal to their argument.