I have enjoyed Bart Ehrman's books in the past, but I'm not sure if can fully agree with him on this point. (I should point out that I haven't read his latest book yet.)
There is certainly enough evidence to suggest that Jesus may have existed or may have even probably existed, but to make a firm statement that he definitely existed is a little extreme in my opinion.
I don't think we know nearly enough about various first century sects and their writings to make definite conclusions about their motivations for telling and retelling the Jesus stories. There likely was a lot of dishonesty, exaggeration, and possibly telling stories with moral lessons that weren't meant to be taken literally. Many, if not most, of these writings may be lost forever. We also might not have any surviving information from some of the groups that contributed to these stories.
What if more than one person or some combination of real and fictitious characters contributed to the Jesus stories? I don't think we can rule this out. In this case, would it be fair to call Jesus a historical person? Would it be fair to call Paul Bunyun a historical person? Some of his stories may have also been inspired by one or more real people.
Would it be fair to say that Big Foot is real? The total amount of evidence for Big Foot far outweighs the amount of evidence for Jesus. The problem is that none of it is particularly reliable and most of it can be interpreted in more than one way. I think the same problems exist for the evidence for Jesus.