we are a startup game studio in Korea
Are you a business (i.e. "LLC", "Limited Company", etc.), or a partnership / group of friends with no legal business status?
Most of us lived and worked in the States back in the day to understand that there are Fed and State taxes to be paid separately in the States.
Not necessarily.
The U.S. has very unusual tax rules about its citizens having to pay US taxes even when they are permanently living/working abroad. If you are Korean citizens who have previously lived/worked in the US, does that apply to you? Or are you US citizens living in Korea? In any case, I would not expect the US to attempt to tax a Korean business with no US presence whose owners happen to have previously lived in the US. But I am far from an expert in this. You should probably talk to an international taxation expert. And make sure whoever you talk to actually is an expert in this specific subject! Don't rely on random people on the internet!