I was pretty good at Chess myself growing up, consistently got trophies in tournaments. Ranked in the top 5 nationally (within the US) for my age group, the trophy I earned for that was bigger than me (I was 12 at the time).

Still though being top 5 in the US for that age group dosent mean very much, the US is one country out of many others. On top of that as you stack against other ages especially say when I became an Adult there is no guarantee I would have even made it to the top 10000, never-mind the top 5, because far more Adults play chess.

I dont think the author is discouraging people from doing that if thats what they are passionate about.

Far more importantly I think he is giving "regular" people who arent gifted a path to massive success by being "pretty good" at a few different things. He is mathematically trying to prove that it will lead to great results, perhaps maybe even better results, than being the absolute best at one thing.

There are diminishing returns to pushing yourself if you arent gifted, you may try your heart out and never ever get to be the very best at Chess. Let's say you hypothetically committed large chunks of your life to it, dont have any other skills, and now are broke and cant live a good meaningful life because you dont earn much for being great at Chess, you only earn if you are the very best.

His path is safer for most of us.