The scene can basically be viewed as a response to, and affectionate rejection of (or alternative to) C.S. Lewis’s religious beliefs as some have seen them to be portrayed in The Last Battle.
It’s worth noting that Gaiman does not see himself as trying to criticize Lewis, but more trying to come to terms with his own feelings about the matter:
NG: Right—I think people who read that story as Neil telling off C. S.
Lewis are kind of missing the point; people who talk about it being
about how we process children’s literature are closer to it. The
actual problem of Susan in the C. S. Lewis books is a moment that I
find deeply problematic . . . you have this weird moment that just
seems wrong.
Fragile Things: An Interview with Neil Gaiman
That said, it seems clear that the scene is meant to present an alternate interpretation or theology that differs from that of Lewis.
The scene is certainly a little obscure, but with some probing, the basic point is pretty straightforward. While the sequence with Aslan and the Witch may be the most attention-grabbing, the scene with Mary Poppins Brings in the Dawn is perhaps more illuminating:
The professor’s lips prickle with shock. And only then does she
understand that she is dreaming, for she does not keep those books in
the house. Beneath the paperback is a hardback, in its jacket, of a
book that, in her dream, she has always wanted to read: Mary Poppins
Brings in the Dawn, which P. L. Travers had never written while alive.
She picks it up and opens it to the middle, and reads the story
waiting for her. Jane and Michael go with Mary Poppins on her day off,
to Heaven, and they meet the boy Jesus, who is still slightly scared
of Mary Poppins because she was once his nanny, and the Holy Ghost,
who complains that he has not been able to get his sheet properly
white since Mary Poppins left, and God the Father, who says, “There’s
no making her do anything. Not her. She’s Mary Poppins.” “But you’re
God,” said Jane. “You created every body and everything. They have to
do what you say.”
“Not her,” said God the Father once again, and he scratched his golden
beard flecked with white. “I didn’t create her. She’s Mary Poppins.”
“The Problem of Susan”
It’s important to understand that this is a commentary on the following dream vision (the one with the lion and the witch). God the Father’s “golden beard” recalls the golden color of Aslan, to start with. What this presents is a counter or alternative to Lewis’s conception of God. The original Narnia books put forth a Christian conception of an all-knowing and all-powerful God in Aslan. In the (non-existent)1 Mary Poppins Brings in the Dawn, God is instead proposed as multiple. There is an entity, in the form of Mary Poppins, that is separate, coeval, and similarly powerful.
This fits with the first dream, which shows an alternate ending (the true ending, perhaps, in Gaiman’s story, where, instead of killing the White Witch, Aslan argues with her over terms):
All this was snow, she thinks, as she looks at the battlefield.
Yesterday, all this was snow. Always winter, and never Christmas. Her
sister tugs her hand and points. On the brow of the green hill they,
stand, deep in conversation. The lion is golden, his hands folded
behind his back. The witch is dressed all in white. Right now she is
shouting at the lion, who is simply listening. The children cannot
make out any of their words, not her cold anger or the lion’s
thrum-deep replies. The witch’s hair is black and shiny; her lips are
red.
“The Problem of Susan”
Instead of being portrayed as a sinful rebel against God, a Satan stand-in (the perspective of the original books), the Witch is viewed as a power equal to Aslan, one who can negotiate with him, and gain an equal share of the spoils:
In the dream, the lion and the witch come down the hill together. She is standing on the battlefield, holding her sister’s hand. She looks up at the golden lion, and the burning amber of his eyes. “He’s not a tame lion, is be?” she whispers to her sister, and they shiver.
The witch looks at them all, then she turns to the lion and says,
coldly, “I am satisfied with the terms of our agreement. You take the
girls: for myself, I shall have the boys.”
“The Problem of Susan”
The use of the line “not a tame lion” here is telling. Aslan is not morally superior to the Witch, in this story’s conception: he is a similarly unknowable and amoral entity. As Neil Gaiman said in an interview:
NG: It seems to me that one of the most interesting things about God as a
concept, if you decide to believe in God, is that God’s ways are
unknowable. And God obviously, look at the world around you, does or
is responsible for some terrible, terrible, awful things. A young girl
kidnapped and kept in the darkness and sexually abused. The deaths of
six million Jews. A mudslide that buries a village. All of these
things. If God is doing the good stuff, he’s got to be doing that
stuff too. If people are standing up there saying, my football team
just won with help from God, then obviously God just pissed over the
other team. So I’m thinking about that and this analogy running
through the Narnia books, the idea that Aslan is the incarnation of
God and he’s not a tame lion, everyone keeps saying he’s not a tame
lion . . . except that he is a tame lion! He’s really nice! He doesn’t
kill anybody, except possibly some really evil witches who kind of
deserve it. Lions, generally, especially not tame lions, are not
people you want to go off with, because they could eat you. They can
turn on you and they can make life really, really bad for you.
Fragile Things: An Interview with Neil Gaiman
And further, and more explicitly:
They’re not people—they’re lions and they’re dangerous! It’s worth
remembering that Gods, whether they exist or not, are not tame either.
Fragile Things: An Interview with Neil Gaiman
That’s the conception of the divine that he presents as an alternative to that of Lewis: an Aslan who is as dangerous as the White Witch, and a White Witch who is as divine as Aslan.
There’s also another intriguing aspect. Note that the (male) Aslan take the girls, and the (female) Witch takes the boys. Even more notably, Aslan and the Witch then engage in intercourse.
It’s difficult not to view this as an assertion of a sort of pagan female/male duality (recalling the concept of the Lady and Lord), over the monotheistic divine/sinful duality that is perhaps more characteristic of the original novels. Rather than being a Satan counterpart, the Witch is then viewed as the divine female counterpart of Aslan.
1: It’s interesting to note the wording in the passage about this book. Not something that Travers never published while she was alive, but something that she “had never written while alive.” Perhaps something that she wrote after she died?