Self-confidence will only help you in games where other factors already favor you.
A charitable reading of the article would be that the other factors are already okay.
Maybe not good enough to guarantee a 100% victory, but let’s say they give a 20% chance of victory (but only if the writer believes in their victory, otherwise they cannot find enough motivation to train properly), the gains of victory are huge, and the costs of training are relatively low. So it could be instrumentally useful to believe that victory is almost sure. (Of course, with all the caveats and proper compartmentalization. For example, author shouldn’t make bets about their victory. Only to believe in the victory while training.)
A charitable reading of the article would be that the other factors are already okay.
Maybe not good enough to guarantee a 100% victory, but let’s say they give a 20% chance of victory (but only if the writer believes in their victory, otherwise they cannot find enough motivation to train properly), the gains of victory are huge, and the costs of training are relatively low. So it could be instrumentally useful to believe that victory is almost sure. (Of course, with all the caveats and proper compartmentalization. For example, author shouldn’t make bets about their victory. Only to believe in the victory while training.)