I have asked myself whether Origen was possibly responding to a question of historicity here:
"CHAP. XLVII.
I would like to say to Celsus, who represents the Jew as accepting somehow John as a Baptist, who baptized Jesus, that
the existence of John the Baptist, baptizing for the remission of sins, is related by one who lived no great length of time after John and Jesus. For in the 18th book of his Antiquities of the Jews, Josephus bears witness to John as having been a Baptist, and as promising purification to those who underwent the rite.
Now this writer, although not believing in Jesus as the Christ, in seeking after the cause of the fall of Jerusalem and the destruction of the temple, whereas he ought to have said that the conspiracy against Jesus was the cause of these calamities befalling the people, since they put to death Christ, who was a prophet, says nevertheless--being, although against his will, not far from the truth--that these disasters happened to the Jews as a punishment for the death of James the Just, who was a brother of Jesus (called Christ),--the Jews having put him to death, although he was a man most distinguished for his justice..."
http://www.earlychristianwritings.com/text/origen161.html
Since we don't have Celsus we only have this oblique reference to Josephus testifying to the existence of John the Baptist and a certain James, who Origen then tries to tie to Jesus.
I've wondered whether this is an early example of 'historicism' whereby the accepted existence of some characters mentioned in devotional texts is expected to 'rub off' onto Jesus by association...
The sense of it might be this:
Celsus, you accept the existence of John the Baptist, right? Well, Josephus the historian mentions John and also James, and we say John baptized Jesus and that James was a doctrinal brother of Jesus. Therefore Jesus must have existed as well.
Is it possible this is what Carrier refers to?