They get taken out on-panel, actually. Imra senses him, but can't pinpoint him fast enough to prevent his ambush. And this is his museum, his time period, and his people's magic spear. He's got every possible home court advantage, and minimal page count to use them in.
Though that's an interesting question now that you mention it. I don't recall if we ever saw any indication of whether the Navy stuff Sword got into was magnetic or not. If it was, lucky for him he picked Cos to ambush first.