It's not like this came out of nowhere either. It's been set up many chapters earlier that MC's father must have been given a powerful elixir when he was young, likely before he was orphaned. Also, he's not so much stupid as he's been a normal man for at least 3 decades. He has no experience with combat tactics or the violent politics of murim.
The storytelling is competent, even if its not to everyone's tastes or reading comprehension.