I was on that flight last Friday.

Multiple reasons for delay:
- Boarding takes for ever with all the document checks for South Korea. Everyone (including transits) need to have all of their documents checked (Covid stuff, applicable visas...)
- Weather, I am not an expert but the flight was bumpy suggesting strong headwinds. In fact, the flight showed an arrival delay way before departure.
- Russia

We got into ICN in late afternoon and the place was deserted which made it easy and quick to go through security (for transit) but documents were thoroughly checked again.

According to current regulations, you can transit without having to worry about SK covid regulations if your transit is less than 24 hours.