That's the point of the test condition. When running a robot becomes more economical than paying full-scale humans $17/h, something important about robot abilities will have changed.
The big win would be training the folks doing stowing to not create such situations and to put markedly different things in each rainbow bin.
Your last point is also interesting given perhaps a robot is more amenable to such instruction, thus creating cascading savings. Each human has to be trained, and could be individually a failure. Robot can essentially copy its "brain" to its others.
Or likely more accurately, download the latest brain trained from all the robot's aggregate experiences from the amazon hivemind hq