She has more than a basic understanding of mythology. (She studied Greek and Roman mythology at Exeter for three years.) She used that understanding to create, as djjd says, her own universe with its own mythologies.
But yes, there's definitely a big element of the Christ myth -- the chosen one, the boy who lived, etc.
It's a pretty universal myth that appears in many different cultures.
That said, it's a children's book. What I like about it are the levels -- there's a pretty sophisticated undercurrent of universal archetypes and lots of fun references/ allusions, but then there's lots of kid-pleasing frippery on top, too.