Tolkien uses the words in a funny way, but what he actually means to say in those letters is that he rejects other people feeling the need to absorb the authorial intent. He wants people drawing their own conclusions in relation to the work, he doesn’t want to prescribe the one correct reading of the text. That’s what Tolkien views Allegory as

I cordially dislike allegory in all its manifestations, and always have done so since I grew old and wary enough to detect its presence. I much prefer history – true or feigned– with its varied applicability to the thought and experience of readers. I think that many confuse applicability with allegory, but the one resides in the freedom of the reader, and the other in the purposed domination of the author.

Tolkien was for sure writing from his experience with war, he just doesn’t want you to have to read the text that way. He wants you to interpret the story however you like.