Tricky
Briefly immortal
Yes, he could have been wrong about "knowing". If there was the possibility he didn't really know, then he shouldn't have said "we know". Saying you know something when you do not could be construed by many as a lie concerning the certainty of your "knowledge". Its a moot point.If you draw parity between those two terms, then we're in agreement.
My problem with the whole assumption of nefarious intent is this: For Rumsfeld to have lied, in the true sense of the word, he must have known something contrary to what he was saying. That means had proof that there were no WMDs, rather than evidence that there were.
What is not moot is that he postured himself as possessing knowledge, not evidence. There was some evidence for WMDs and some against. For example, the UN inspection tours had not found anything of substance and strongly advised continuing the search rather than invading. So we started a war based on "knowledge" that had been contradicted by others, including some of our allies. If that wasn't a lie, it was just as bad. In fact, it was worse than most lies because of the results.
Call it what you like.