I'm using Chronicle, as the most recent telling of the event, as my source (since Blizzard tends to default to 'the most recent retelling is the canon one' as their go-to, and with the WC3: Refunded 'updated cutscenes' not coming to fruition, that leaves Chronicle as our most recent, up-to-date source)--and as Chronicle doesn't say Arthas disbanded the order, despite him declaring such, and instead only sent them away and considered it treason, it stands to reason he doesn't hold the authority to disband the Knights of the Silver Hand, and Terenas overruled Arthas's accusations of treason.