I bet it’s because of him writing. He probably has had a character who is rich and knows about stock trading. He probably does a lot of research for his novels.
Now this makes sense!!! He's not just a fast learner but he also could retain knowledge damn I wanna be like him
i dont think ideum talked to him about stock trading. ideum even reacts with "did i?" because he doesnt know what jiseong meant. kwon jiseong was most likely talking about the first time they bumped into each other. jiseong was contemplating whether or not he should sell now or wait it out, then he overheard ideum saying "i should make a run for it". so im assuming jiseong took that as a sign and chose to sell and got a better deal. jiseong believes about his luck in stock trading. hes basically thanking ideum for bringing him luck and not that ideum actually adviced him.
Ykw it's kinda shocking that ideum knows about stock trading but please don't make the big bro be more attached to you