"Holding Torque" is the torque when the motors are not spinning.
What you really need is the torque at max speed.

I keep meaning to read up more on why metal cutting machines use much larger stepper (or servo) motors
The simple answer is that's a general assumption. A properly machine will have a motor that provides the required torque to meet the performance goals of the machine, plus a certain amount of headroom so the motor won't stall or lose position. This may be 50% more than the minimum required torque.
A big router will have bigger motors than a small milling machine. It all depends on the specific machine.