I had this hotel offer included in a FRA-ICN-SYD-ICN-FRA trip. As the SYD-ICN and ICN-FRA connection needs one night in ICN OZ provided a hotel voucher. I cannot say anything about the quality of the hotel as I chose to stay at the airport Hyatt instead on my own dime.
The hotel was part of the J fare which also came with a one-way upgrade to F on the FRA-ICN leg.
The transit hotel in the terminal at ICN is also quite ok. I used this on the outbound trip, as I also had about 8 hours until my connection.