I think Yardley is referring to the idea that film grain is thicker and more uneven than the photosites on a digital sensor, and that film therefore captures (marginally) more of the "spread" of the focus and so gives the impression of more depth of field.
I don't know if this is true. Sounds somewhat plausible theoretically but like many things, I've read about it on the net and never seen it demonstrated.
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