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The author seems to have limited himself to five digits as opposed to more, when he should have also addressed the 5 or less argument as well..

Experiments in robotics have shown that 3 digits is minimal..
Two "fingers" and an opposable "thumb"..
One thumb and one finger will allow grasping of an object, but control or manipulation is "shaky" as the object will rotate (laterally?) between the two digits...
digit #3 allows control of that rotation..

So one could argue, that for the sake of efficiency, humans could get rid of the 4th finger (pinky) and still have sufficient digital dexterity for any imagined task...

In many animals, an extra digit remains in the form of a "dew claw", which may be a disposed of thumb..
In hooved animals, the digits have evolved into 2 digits, (cloven hooved, like cattle and deer) and 1 digit ( hooved, like horses )..

The author looked in the wrong direction.. evolution "simplifies" ...
In different orders, digits have evolved to the task at hand..

Oh.. and then there's "flippers" on seals, etc.. and the whales... Once a land mammal, now sea-borne, and still retaining vestigial digits within it's flukes..