I see you read my post carefully. At least I am not the only one slow to catch up. BTW - you would not hurl anything. Hurling something, in other words, imparting momentary instantaneous acceleration, would result in a constant speed in a microgravity environment. Nothing we know of could possibly impart anything even close to c on such a large object with a "hurl".Speed is not the issue... it's speed/distance/time....An object in motion tends to stay in motion; an object at rest tends to stay at rest - it is the "law" of inertia and applies whether or not the "object" is in a gravitational field. https://en.wikipedia.org/wiki/Inertia Therefore, any object would experience the effects of acceleration and deceleration regardless of whether it was in a gravitational field or not. With schemes like this, an acceleration of 1G would produce a gravity like effect for anyone inside the craft, and applied over sufficient periods of time, could allow a craft like this to reach the speeds they cite in the article. I'm not versed on the specific instance, so I do not know what their planned acceleration is.
As others have noted, the laser idea is nothing new. It has been around for ages and relies on imparting a constant acceleration. It is the same concept that made Deep Space 1 so successful.