Couldn't agree more... do not head to the concourse terminal too early.
Just fyi, there is now a maglev operating from Incheon airport to an area nearby. It is a total of 6 stations and as of yesterday, the ride is free. Operating hours are 9am to 5pm. This might be a way to kill some time. Give yourself an hour to ride it to the end and back. Not much there, but some nice views of aircraft landing (depending on wind direction) and also there is a water park operating nearby. The maglev is located above the ice skating ring near the airport express train entrance.