When I first visited Seoul, I thought so too, "worst traffic in the world". But after visiting Bangkok, Seoul's traffic looks wonderful.

I think maybe Skchin was referring to not many cars on the road between ICN and SEL. There are plenty of cars but nothing compared to in the center of Seoul and plenty of lanes to handle the traffic without ever a delay unless there's and accident or other incident. Thus a ride on the bus or a ride on the train takes about the same travel time. But with a much further walking distance to/from the train, and the waiting time for the next train, a bus is the faster of the two options.