No, that's a completely unbelievable explanation. If he was fictional he'd not have been of Nazareth with a circuitous reasoning about why he was also of Bethlehem -- he'd have been directly of Bethlehem. His name wouldn't have been Jesus with a completely circuitous explanation about why "Emmanuel" also counts as his name, his name would have been directly Emmanuel.
These all seem to me to be false dichotomies, which assume that it's impossible either for a single creator to have embroidered their story as they went along, or for multiple creators or editors to have changed the story at different points in time.
Nor do I know of any fictional characters that are so deliberately placed recent history and yet their existence is believed by their contemporaries as real. If the Christian movement had began in the 1st century, and yet its founder placed as having lived in 3rd century BC, that explanation might make sense. But he was placed as a contemporary, and expected to be believed to be real. You don't do that with fictional founders of your order.
As long as it's far enough away in time and space that your claims can't be checked, what difference does it make? This seems to me like a post hoc justification for believing the Bible story, not an argument that anyone would have come up with if they didn't have a pet hypothesis to defend.
Also we don't have any evidence that Jesus' contemporaries believed he was real. The reports of people believing Jesus was real come from long after Jesus supposedly died.
These all seem to me to be false dichotomies, which assume that it's impossible either for a single creator to have embroidered their story as they went along, or for multiple creators or editors to have changed the story at different points in time.
You know there's no impossibility -- that's why I said "I assign less than 5% probability", I didn't say "I assign 0% probability". I'm talking about the probabilities of each scenario. Conditional and prior.
If you considered that, and you didn't treat "completely fictional" as ...
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).