There are ... speculations that King Sejong was not fully independent in the creation of Hangeul. He may have delegated parts of the task to courtly scholars. (But all the evidence either way has been lost to history, and so speculation it remains.) There are a few "historical" dramas which make this their premise.
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.