1. Become the best at one specific thing. 2. Become very good (top 25%) at two or more things."
I'm certain at least some small part of my own success can be attributed to my exposure to this idea, and for that I give my respects to Adams. As far as Adam's character (or lack thereof) is concerned, that's already being discussed elsewhere in this thread by others more eloquent than myself, so I'll leave that to them.
Is this idea that top 25% is "very good" at something innumeracy, or a subtle insight I'm missing? There's got to be a million skills that you could assess rank at -- writing embedded C code, playing basketball, identifying flora, PacMan, archery, bouldering… I can't imagine ever being able to not continue this list -- and you should expect to be in the top 25% of roughly a quarter of those skills, obviously heavily biased towards the ones you've tried, and even more biased towards the ones you care about. It's hard to imagine anyone who's not in the top 25% of skill assessment in a dozen things, let alone two or more…
Everyone (for the sake of my argument) wants to be an engineer at a FAANG but there are tons of folks making more money with more autonomy because they've found a niche that combines their good-enough technical ability with an understanding or interest in an underserved market.
Beyond that, it's not about becoming very good at two different, completely orthogonal things, it's about becoming very good at two things that are complementary in some way that is of value to others. Being good at PacMan and Bouldering is only particularly valuable if you are competing for opportunities to participate in a hypothetical mixed reality video game, or perhaps a very niche streaming channel. Being the top quartile of embedded c code, and flora identification could result in building software/hardware tools to identify flora, which is a niche that currently has multiple competing products that are high value to those interested.
If it still sounds easy, try to reach top 25% rank of a video game that you are not familiar with (diamond in Starcraft II or whatever). You'll find it's literally the workload of a full-time job.