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.
We're in a agreement then. I understand the difference between allegory and metaphor, but some people understand those terms are being interchangeable, even though they aren't. You are 100% correct.