The only exemption I saw in the FAQ for IKK was:
² There are some technical details. There can be J seats showing for sale, but the flight might be oversold in Y by such an amount that exceeds the total number of seats for sale on the plane. In that case IKK is not available.
This doesn't seem to apply - J3 Y9 doesn't sound full in the back, though I can't know for certain.
Perhaps something else is at play?