@Fari It's because he's already learned it. He already knows everything about the skills, how they work, their ranks, etc. He just needs to flip through the book as an unlock requirement.
Even in a game, there's a big difference between using a skill and using it well, especially if it's an SAO-style VR game where your body movements would actually matter. At the top-level of a game, that difference is even more extreme, so I'm certain that getting to his level of knowledge and skills would take at least several months. Regardless of how fast you learn, it would still take a ton of practice, so constantly being berated by people looking for cheats or "the easy way" would be obnoxious at the very least.