What he’s probably referring to in the Roman feast of “Sol Invictus” or the feast of the “unconquerable sun” which was celerated on December 21st or 22nd(the winter solstice). The Roman church did this a lot, providing christian alternatives to pagan festivals.
Why didn’t the Roman church instead go with Clement of Alexandria, who gave a first-hand account of the evidence he had at his time concerning what Christians before him believed, which he listed as March or April 20 or 21 as the most likely date(s) for Jesus Christ’s birth?
This is what Christians believed before Clement, and he gave testimony to that.
Clement would have been a much better source to rely upon than anyone else, and thus any chance of historical revisionism and then a celebrating of Jesus’ birthday on any other month or day other than March or April 20 or 21 would have been avoided.