They sound very similar in Chinese, and in China people usually refer to Incheon airport as "the Seoul airport" (a lot of average Chinese people probably don't even know Seoul has another airport), and to make things worse, the IATA airport codes for the two are INC vs. ICN.
So the urban legend is that for a lot of Koreans living in Beijing, when they are planning to go back to Korea, they go to a travel agent on the street (this was around 2000 so online booking were not ubiquitous) using their not so fluent Chinese to say "I need a flight ticket to Incheon", the agent will often book them to Yinchuan instead. Korean Air had a direct flight between Yinchuan and Incheon for a while (they probably still have that flight) and the urban legend is that flight is mainly for those confused travellers.