This is one of my favourite essays because it completely changed the way I thought about my own skills. Namely, it taught me that a good way to evaluate my own skills is to examine the gap that exists between my skillfulness in some given field and the skillfulness of people who are not in that field or are novices within it.
My takeaway from the essay was not that athletes are vapid or unaware of how much more skilled they are than laymen or amateurs.
My takeaway is that when you're really good at something, to where you can say you're better than a lot of people at it, you're often not aware of exactly how good you are at it and may even see it as banal. Where other people might look at some skill you have and be blown away and wonder what it's like to be so good at something, you just recognize it as your default and unremarkable state.
An analogy I can think of is literacy. Many years ago, I taught myself literacy in a non-Latin script for a language that I had grown up speaking but had never actually learned how to write. Reading in a new script was incredibly slow at first, as I would literally have to sound out every individual letter in my head, and then manually put them together in my head to understand the word. A sentence would take me minutes to read. Over time, I would see words that I had read many times and I wouldn't need to sound out the letters in my head anymore, I would simply "recognize" the word: my brain would recognize the collection of letters as an image associated with a concept.
I realized that I had always done this with English and had never been aware of it. When I read English, I'm not really reading each word as a collection of individual letters; I'm reading each word as an image for lack of a better term that I can immediately associate with meaning in my head, and I think this is how most people read English.
How this is relevant to this essay is that if you were asked about your ability to do this, you would think it something completely banal and regular. You might not even have any particular comment to make, as you'd simply see your literacy as your personal status quo. You've spent most of your life actively training your skill of literacy and have attained mastery, but to you its...just reading. Suppose you talked to someone barely literate, or someone learning literacy in English, about this ability -- to them this ability would be much more impressive because the gap between their skill and your skill is much wider.
I think realizing this about yourself has all sorts of applications: confidence in your ability lets you experiment more or take action when you have less info or security on the outcome of your action. Realizing what comprises a given skill gap would help you teach others how to get to where you are in a way where you teach them at their level, not yours.