It's true, because it's not a Christian. [i.e. the pie]

The story:

I thought it was a reference to witch-burning! The verse goes,

"The food was great, the tree plugged in, the meal had gone without a hitch
Till Timmy turned to Amber and said, 'Is it true that you're a witch?'
His mom jumped up and said, 'The pies are burning,' and she hit the kitchen,
And it was Jane who spoke: she said, 'It's true, your cousin's not a Christian.'"

According to my misunderstanding, Jane is wryly joking that the pie is burnt because it's not a Christian.