To be fair, he seemed to be in his 30s or so there and even though he might have been the best already at the time, he was also cocky and hadn't been knocked down a peg or two yet as we see at the end. This screw up with the book was probably the first humbling experience that eventually made him truly great or at least teach his grandson how to be the greatest.
I'm going to use myself as an example. I'm the only person supporting a custom piece of software my company uses, and I've been doing so for over a decade. I've designed and tested new features, troubleshot numerous issues, and provided regular training classes for our new hires. The only person who knows the program as well as I do is the developer, and that's only because he can actually dive into the code.
Even with all that knowledge and experience, I've still been humbled a couple of times when I was
sure I knew what was going wrong and jumped to the wrong conclusion instead of actually listening to the person trying to explain an issue to me. I'm just fortunate that my mistakes could be caught before I did anything more than waste time.
So I can absolutely believe that a guy who's become so successful that even the King is sending him work could make a fuckup like this. He knows he's good and it's lead him to jump to some stupid conclusions like "I don't need help", and the resulting clusterfuck is going to completely change his worldview.