He implies they are in possession of the weapon because they were on the nautiloid, but he never really gets the chance too see the weapon, or doesn't even hint to being able to read Lae'zel's mind.
He just thinks, they must be holding the weapon but he can not be sure of it either and for what is worth to him, killing them costs him nothing and if they reveal to not have what he seeks, they are just other casualities that fell in order to realize his queen's order.
I admit the scene could use some more polish, but I can also see why he is leaving.
Besides if he is to join the fight, we might encounter them at a higher level, with the actual possibility of defeating them.
Otherwise better to rework the scene completely, even though I don't find a major flaw in it as you do, not until we get to know more about kithraak's motivations and true intentions.
I can see where you are coming from, I think they need to make more clear the power of the artifact that the party is holding.
