[improbable22]

No, not really. Just about everything we say about black holes is talking only about the exterior, the region of spacetime outside the horizon. The simulations through which we model the merging of two black holes only model the exterior of both. The curvature of this exterior is what gets all churned up during the collision, and some of it ends up travelling off as gravitational waves.

The fact that we can get away with studying only the exterior is really the same fact that the horizon is a horizon. The causality only goes one way, for everything not just for normal matter. And thus ignorance of the interior is no barrier to understanding the exterior.

[mirimir]

The article says basically: 7M BH + 12M BH -> 18M BH + 1M GW. But I'm getting the impression that this is misleading. Wouldn't it be more accurate to just say that the system had 19M before merger and 18M afterward?