If their occupancy ratio was lower, then there wouldn't have been a problem, they could have fit the crew in. If they reserved 4 seats for crew on every flight, again there wouldn't have been a problem.
If they had even reserved 2 seats and run sufficient flights such that the crew got to the target airport in time there wouldn't have been a problem.
If they had brought crew in from two different starting points - it would have been fine.
It's still overbooking, just not oversold tickets.
The airline played the game, ran too lean and paid the price.
Much like an ISP having all their IP traffic going down one pipe, and not having enough room to send out the customer invoices because everyone is watching Netflix.