Now that's an Interesting line of thought. When I started to dissect it a little more, it's obviously all laid out there in the Aslan's camp scene (and that's bookverse and movieverse, both) so the army knew, but that does sort-of beg the question as to what percentage of the Narnian population fought (as opposed to hiding/being turned to stone/refugee in Archenland/etc.). I've read quite a bit of fic that has the betrayal as an open secret, but your point about the rescue and not talking about what was past strikes me as equally legitimate. It's a fairly fine distinction, but certainly plausible. And ten years on, memories could well have blurred significantly. I buy the scenario that you've described from the head canon, and I certainly buy the premise of the Islanders looking to the rational to explain away the mystical.