Certain seats are reserved for airport check-in and certain seats can only be reserved by Medallion level fliers, so the flight is not necessarily overbooked, although it may be.

But overbooked is far different from oversold..it is very typical for a flight to be booked > capacity, never sell out, and depart with empty seats.

I wouldn't be too concerned, although those passengers without pre-assigned seats are prime candidates for involuntary bumping. As discussed above, the chance of that is very, very low...