I was reading the manga for that one, it at least has a plausible explanation of where the skill trees for different classes are largely unknown and any discoveries are kept secret. The MC has the advantage of knowing all of the skill trees and what combinations will basically make a class “broken”
He was mocked and disowned for choosing the heavy knight because the class is known to be horribly flawed, but he knows how to add a counterpart skill that fixes the main flaws of the class and making it overpowered. He does the same thing for his closest ally, who had chosen a class that has a high level of luck and he helped her get a skill or item that has a massively strong critical hit (which happens frequently due to her high luck stat)
It’s reasonable to expect him as a gamer to have read all of the guides about the skill trees and available items, while most people in the world would have to discover the secrets of the skill tree by trial and error - and they would completely disregard classes and skills that seem useless because they can’t risk exploring the skill trees as they cannot change their class or skills after they have been chosen. If they pick a skill and it’s bad they are permanently fucked, they might not know that it becomes good down the road and they might make a wrong turn somewhere and completely miss the overpowered skill
