Before this I too thought "waterproof" and "watertight" were synonymous, but apparently in marketing lingo they are different (I wonder if there's a gov't standard for watertight). Regardless, from the quoted section above, watertight has something to do with "it floats". The question that comes to mind is, "floats for how long"? It probably floats long enough to grab it before water seeping inside causes it to not float.

In your case with it being a carry item, I would look at a small dry bag. It says "water-resistant" so again, I have no idea what that means regarding being dunked.