Probably it is just designed to not get water inside it (wouldn't want your wings filling with water while flying), so it happens to float.

One thing that is not clear to me is if this part is mostly aluminum or mostly carbon fiber. If it's carbon fiber, the fact that it can float might not be so surprising. Another interesting question is just how high it was floating in the water. If it just barely reached the surface then it would experience very little windage, so its path through the ocean would be almost entire driven by currents. I don't know that that would simplify drift analysis, but it seems like it might.

While I don't doubt that it is sealed, I wouldn't think it would have to stay completely sealed to float: I'd expect that a sufficiently large air pocket trapped iinside could do the trick of keeping it afloat just enough for it to travel somewhere.

Don't they have fuel tanks in the wings too? That might be a reason to have it completed sealed up.

Maybe so it can be found in the event of a crash :D