If you had taken my advice and not watched the video, you would not now have this wrong idea stuck in your head.
No, we're not assuming he was real from the get-go.
However, when we run across correspondence from a group of Jews who say they're followers of an apocalyptic holy man named Jesus who died recently, then historically speaking, the most likely explanation for that claim is that they're followers of an apocalyptic holy man named Jesus who died recently.
That's not problematic. There's no problem to be solved by proposing that they're wrong.
When we discover that they're having to explain away embarrassing facts about this guy, such as his coming from Nazareth, being baptized by John (in light of their claims about him), and his being crucified… and when we consider that there's no precedent for any such literary or religious figure for them to draw on… then that likelihood goes up through the stratosphere.
Now, one problem about the scenario you describe above is that Paul already sees Jesus as a human being, born a Jew, born of a woman, born into the Law, flesh and blood, crucified, dead, raised from the dead. So there couldn't be any conversion of concepts of Jesus from divine to human after Paul, since it must have already happened before Paul.
Moreover, while Paul records many disputes between him and James the Brother of Jesus and the members of the Twelve in Jerusalem, he reports no dispute over this issue with them. (Although he does take issue with some outside the circle of those who knew Jesus.) Which means that it's most likely that they didn't disagree with Paul, which means the earliest Christians thought Jesus was human.
Finally, there simply is no tradition to be cited that would support the "Jesus as divine non-human" scenario. What, exactly, does this guy think they were drawing on? Is he keeping some ancient manuscripts to himself?
Keep in mind, these folks didn't say Jesus was some ancient figure. They said he was a contemporary holy man.
I don't know of a single instance of any group claiming that they were recently founded, just a few years back, by someone who never existed.
Can you find a precedent for that? Hopefully one somewhere near the time, place, and culture we're discussing?