The difference is that in that direction, the connection time is 80 minutes, which is more than the MCT. So you can actually ticket the connecting flights on one itinerary in that direction.

In addition to not being able to ticket it on one ticket ICN-NGO-GUM, if the agent at ICN is on the ball, they will refuse to accept through-checked baggage even on two tickets, because of the MCT issue. So, the OP may be able to actually make the connection if the inbound flight is on time, but will not be able to check bags, will have to buy two tickets (perhaps at a higher price), and will have no recourse if they mis-connect.