Overbooking for airlines is nothing new... They have a very complex software system which determines how many to overbook and it predicts how many will be "no show" so offering a ticket on a sold out Y class is not big deal because they can always bump people up to J to make room and if let's suppose Y class fare is $2500 compared to cheapest being under $1000 . It would makes sense to show red flag to bottom ticket bucket holder.

For this very reason airlines often sell full fare tickets at the airport counter.