You either read wrong, or someone explained wrong.
Your seating assignment is given to you on your sail&sign, which means that they have to have the seating assignments before the sailing day.
If they are like other organizations which do similar things, they will run a best-fit program against the passenger manifest once they are 99.5% [I'm making that number up; it is to signify "the vast vast majority"] booked and get seating assignments for them all. I would guess that they have the seating assignments no earlier than a week before sailing.