"Regardless of whether this was Christ's actual birthday, the symbolism works. And Christ's birth is inextricably linked to His resurrection."
Bingo. Christ's birth, Christ's resurrection and Christ's relationship with mankind is what matters. Who cares if we got the date right? I never remember my wife's, either!
That is the point of the article. Veith isn't saying with absolute certainty that December 25 was the actual date. (Note the "Regardless.") What he is saying is that the ancient belief about a death-date/conception-date congruence--in this case, a March 25 Crucifixion/Annunciation, making for a December 25 Nativity--works well theologically in connecting the Incarnation with the Passion of Our Lord. Furthermore, a December 25 date serves the theological point of the darkness not being able to overcome the Light of the World.