That's a very common trope with prophecy - something that appears to obviously read one way, but then turns out to have had some twist to it that when interpreted differently actually changes everything.
And by the way: this ambiguity doesn't exist in many other languages, and we can be pretty sure he didn't say it in English originally. Tolkien the linguist would have been acutely aware of it, and the ambiguity of the English rendering likely exists PRECISELY in order to blur the distinction which, in the original language, probably wouldn't have been there. It makes us, the reader, all the more aware of the ways in which language tricks us, something that a linguist author no doubt would find immensely gratifying.