I guess what I would say is that it is not an allegory for Christianity and Christ. It actually was a story ABOUT Christianity and Christ. But I guess it depends on how you define allegory.
I have to agree and disagree with you. Allegory has a definite meaning. It doesn't depend upon individual whim or taste. Allegory is a literary device in which characters and events stand for abstract ideas, principles or forces, so that the literal sense suggests a deeper, symbolic sense.
Individual interpretation of a piece of writing is, however, a personal judgment. Is the lion symbolic? Yes, a symbol of Christ. Is the lion symbolic of an abstract principle like justice? It could be interpreted that way. But from the letter you quoted, Lewis wasn't trying to create a work to convey abstract concepts. The lion is Christ. That would be metaphor, not allegory. If the story is interpreted as the triumph of good over evil, that is allegory.