Yes, you would take the train to the main concourse, go through transit security, and then after you are finished shopping take the train back. It's important to take the train first, because if you clear transit security in the satellite concourse, it's not possible to get to the main terminal. If transit security is backed up, it is sometimes faster to enter the country and then go right back through immigration. That said, ICN is usually a very efficient airport for transit.