I think I earned mine. It was the first exotic I got when he came with it the second time around. I didn't really know much about the game and every strange coin I earned were from engrams. I couldn't do the weekly's because none of my friends had destiny. Buying your weapons from Xur is earning the gun mixed with a little bit of rng. You do not earn it from a chest, killing a boss or some pop star wanabe. That's just dumb luck.