This might sound a bit far-fetched but one theory I've heard is that Burt evicted Lizzy to infiltrate the Mallers. If this is the case, he must know that getting her out so soon would be a bad idea. As he was the only one who saw the mall, he may have just said the Mallers were gone to delay a rescue attempt and allow Lizzy to gather more information. Or he may have realised that they didn't stand a chance against the Mallers on their turf and knew there was no reasoning with Saul, so he had to resort to other means to stop the rescue. Far-fetched, I know.