I have no idea why this story is being treated as a big deal. Okay, I know: most journalists know hardly anything about Biblical scholarship, so when the Pope says our calendar gets the date of Jesus’ birth wrong, they fail to think, “Oh, scholars have known this for a long time, and because neither the Bible nor Catholic doctrine makes any claims about the modern calendar, the Pope acknowledging this fact is not a big deal.”
The situation is more complicated than that, though: actually, if you go by Matthew, Jesus seems to have been born in 6 B.C. or so, whereas if you go by Luke, Jesus seems to have been born in 6 A. D. Together, Matthew and Luke cannot be reconciled with secular historical records. Richard Carrier has a good discussion of this issue here. And it isn’t just the little issue of dating that Matthew and Luke disagree on; their accounts of Jesus birth are wildly divergent and impossible to reconcile with each other.
Now if the Pope admitted that, that would be news. Heck, you could probably do an interesting story on this book of the Pope’s just by having someone with a basic knowledge of NT scholarship read it and see how it deals with the irreconcilable nativity stories.