No. While Kariaq Dreadking's units are considered to be of the Astra Militarum faction while Muster the Guard is being played, when Kariaq Dreadking deploys a unit the event is no longer being played, and therefore the unit being deployed is not considered to be an Astra Militarum unit. Thus the discount does not apply.

It is theoretically possible that, if the deployment was taking place during the resolution of an Astra Militarum event, then the unit would be considered an Astra Militarum unit, and the discount would apply. This scenario requires very precise setup, and is unlikely to occur during play.