Read the part that says "no-one paid the supplement" to sit in said rows. Therefore it is down to the crew to move someone.
I check in a loco (as well as the same airline as frontcheck) with this policy, if no-one pays to sit there then the crew are to move an ABP into the seat. More often than not we will endevour to sit ABP directly behind or infront of these seats, this depends on what is available after online check in though.