The failure modes listed in the paper are models with a lot of concavities or self-occlusions. Looking at the video, it looks like the elephant cup succeeds because of surface tension properties of the water, which let the surface on each side of the trunk "snap together" when the trunk begins to be completely immersed. It's that wrapping or stretching that lets it be more than a simple light projection, which would fail due to self-occlusion.

Thanks. I found this in the paper too. Like you said, high concavity and significant self-occlusion are two cases that are more likely to fail.