I think I"ve seen J6 or J7 and no seats available on some flight in the past.
No seats available is not necessarily the same thing as all seats taken.
For a simple example (though I don't know if it could apply to this case), it can be that they don't yet know what exact equipment they'll use for that flight, and the seat maps for the different metals being considered are incompatible. Until they resolve that, they can't assign anyone a particular seat, not even the first person to book that cabin.