Note that we have been attacked with this horn before, so if the previous weilder spoke a command word where we could hear it, then we know what it is.
Well, I've been trying! I found where we detected magic on it, and I found where we were attacked with it, but neither of those posts said anything about the command word, other than that one was said immediately before we were attacked.
Yes, you are right. But maybe kinem isn't intentionally wasting our time, only misremembering what we have been told. Because I found a few places where I fully expected the command word to be revealed, only it was not. And, like I said before, I did find where Detect Magic was used by Bevin upon all of the gear we got from that encounter, including Bev's Cloak and the Horn.