AFAICT DL has been operating routes like DTW-(ICN)-PVG several times per week. It's a direct flight with a "technical" stop at ICN to change crew, who immediately fly back to ICN without entering mainland China. This should not be confused with trying to connect at ICN with different flight numbers, including a different DTW-ICN flight that departs around the same time. This would require a COVID-19 test near DTW, not ICN. In fact, passengers don't deplane at ICN at all, not even to use lounges and certainly not to enter South Korea (ROK). It's not a connection or a stopover at ICN, just a technical stop.