That's not law, it is custom, and for some things it's the published regulation. Yet in any case, if there's reasonable evidence or testimony that contradicts the assumption of proven citizenship given by any document -- such as passport -- it would be unreasonable, if not actionable misfeasence of office to ignore contrary claims.
The Presidency is the most powerful office in the world. You would accept an expired passport for the Presidency?
If I were only 99.9% sure my brakes were good, I'd have them checked.
Is my Country worth less than my car or my life?