I think that they neglect to tell you that this only applies if the same voltage and current is used with both wiring methods.
Stepper motors are rated for holding torque, when the motor is not spinning. Whether wired series or parallel, the motor will have the same holding torque.
Now, as rpm's increase, the torque begins to drop off
If parallel has more torque at higher speeds (which it does), and both start with the same amount of torque, then a series wired motor will never produce more torque than a parallel wired motor, unless the parallel wired motor is supplied less current. Remember that a parallel wired motor has twice the current rating of a series wired motor.
What exactly is high speed and low speed would tend to depend on the specific motors you're using. You'd need to see a torque curve to see where the torque falls off for the motor when wired bipolar series. Every motor is different, and the voltage you're using plays a role here too.
Generally, I would say that you can get away with bipolar series on smaller Nema 23 motors if you don't need much more than 600-800 rpm. If you need rpm's higher than that, then parallel is usually a much better choice.
Motors wired bipolar series may run cooler, due to the lower current.
The bottom line, though, is that switching to bipolar series will not give you any more torque than you have now.