It is said in this line that event supervisors will supply a top anchoring system to attach to the TOP END of the elastic cord. Does this mean that teams will be allowed to use only the full length of the cord for each drop?

No, teams can attach the anchoring system to any point on the cord. Since the top end is defined as anywhere the bungee cord is clamped/attached to the release device, each team in effect determines the top end by where along the cord they fasten the bungee to the release device.