So if it can only go up to 480 mV, even without a load, how is it going to overcome a 1.5 V burden voltage? The Fluke 116 will have a shunt resistance of 3.1 kohm (1.55 V / 500 uA). 480 mV across 3.1 kohm results in 155 uA, pretty close to 158 uA.
My guess is that either the 480 mV open circuit voltage is wrong, Doug misunderstood the Fluke 116 burden voltage spec or was confused about Ohm's law when he answered your question.