For the record, the reason they say that is because many ocarina makers just use a popsicle stick to make the airway of their ocarinas. The exact width of the airway is actually less relevant than the thinness of the airway, as well as its connection to the fipple (the sound window). No need to make precise measurements with fancy instruments when a popsicle stick works just fine.
Source: was an ocarina collector for many years, am generally familiar with how clay ocarinas are made