Here's the OED definition, which may (or may not) help:

allegory n Narrative description of a subject under guise of another having similarity to it.

The allegory is the description, not the subject. I think what was meant was that JC is either

a) a made-up story (therefore not real), or
b) a real bloke (therefore not a made-up story)

Of course I can't know what exactly was meant. I'm only human alas.