A deity is a mental construct, even if you give him a backstory as a historical figure. And the deity and his story aren't really related anyway. An imaginary friend is an alter ego of the believer, which is about their psychology.

The idea that it's somehow driven by the story is one of those things religion takes on faith but isn't actually how it works. And if there was some guy who gave his name to the character in the story, he'd have nothing to do with the story either. And even if there was a guy as the namesake, if you could go back and in time you'd find he's like other cult founders, a con artist.

So there's at least three levels of disconnect, the Jesus in your head has nothing to do with the story, the story had nothing to do with the guy even if he existed, and if he did he was just bullshitter like Joseph Smith or L Ron Hubbard or the guy who came up with the Hare Krishnas.

What's going on with "Jesus existed" is his existence is taken as validation of the imaginary Jesus. Which is a reflection of the believer, so it's really about personal validation. And that works the same whether they're reaching for a divine Jesus or a historical Jesus.