For example, I read that North Korea actually had a bigger GDP than the South at the point of their partition, which is obviously not the case today. Does this mean that the North had historically been more economically important before the division?
During the Joseon times and Goryeo times, Gdp per capita wasn't really a thing. There were servants, commoners, yangban, and nobles. If you're talking about Nominally, than South of Korea was generally always richer, as it held better crop yields and the north has often skirmishes with Manchus and Jurchens.