If you cut the quote short, as is usually done, then your interpretation is actually the most sensible one. If you DON'T cut the quote short, which is the only fair way to assess his statements, it becomes ambiguous, and your reading is still a plausible one. But Rumsfeld himself has already stated which interpretation he claims is correct, and given that there is ambiguity, I don't think there's a basis upon which to call him a liar.
But I would note that pretty much whenever the claim is made that he lied about this, his response is cut short. There are a whole lot of people who simply aren't interested in actually finding out what he really meant, but just want to pin an accusation on him.