I'm not sure this shader was ever intended to get this macro with its effect. I've had some fun and games recently playing with Volume 1 density to get the decay to work into white rather than the default black. Which worked. What you are suggesting here is that there's a detail drop in sub-surface calculations, which is maybe why I wasn't achieving the effect I was looking for.
I also ran into some other issues with using two planes, one distorted with a mask defining a hole, the other offset to the hole wall depth defining a water shaded plane. The two didn't seem to want to work together at all.
But back to your example, it does look as if some form optimisation maybe in play. (?)