In many ancient and medieval monarchies, any major project that occurred with the king's approval was attributed to the king personally, regardless of his level of participation. Kings built plenty of castles, but they probably didn't move a single stone with their own hands.

The academic consensus seems to be that Sejong had a lot of personal involvement in the creation of Hangul, though. He was a quintessential nerd, after all, who didn't have much patience with the conservative officials who got in the way of his nerdy projects.