KE did not always allow seat selection at time of booking. Now there is some language on their website that seems to indicate seat choice is allowed only if the passenger is a logged-in member of their frequent flier program.

I don't want to credit the miles to the Korean Air program. I also don't want to fly without advance seat choice.

Have you booked on their website and gotten seat choice in Y? Did you have to attach your frequent flier number to the ticket to be able to pick your seats? Were you able to change seats later?