It's grammatically correct but the main reason it sounds strange (and it does) is because of the position of "incredibly". It would sound more natural to place it after "economy". The second reason it sounds strange is because we normally talk about the "economy" as the subject of "develop" and without an object. You have to change the sentence to do this. The third reason is that it would sound more natural to talk about the development of the economy with the word "rate". So, in my view, this would sound significantly better. "In the 1960s, Korea experienced rapid industrialization. Although this meant that the economy developed at an incredible rate, it also intensified the gap between the haves and the have-nots." (I've also removed "a" from before "industrialization")

I think it sounds a bit awkward, sorry! Here is how I would suggest fixing it: "Korea experienced rapid industrialization. Although this industrialization lead to large developments in the economy, it also intensified the gap between the haves and the have-nots."

It's not strange in my opinion. In the first sentence "In the 1960s, Korea experienced a rapid industrialization," the word "rapid" is a positive adjective meaning that the industrialization was quick and productive. The part where it says "Although it incredibly developed the economy" still tries to make a positive tone because of the upcoming negative tone the second part will bring ("it also intensified the gap between the haves and the have-nots.") I hope my comment made sense :3