I noticed that about Lab too. I wonder if hitting those with a PB will do more damage too? I still don't get why you can aim at them but they add no additional damage.
Actually the lab stage that you see in the video "Phase 4 VS Bayman" is using the version of the level available during the chapter 69 of the story mode, when the structure of the lab has still to mutate for revealing to the walls the various clones of Kasumi kept inside. https://www.youtube.com/watch?v=4c6cSaFsQIo
People speculate that could be an alternative version of the lab stage available in DoA5LR (just as already happen with the stage "Dead or Alive", just for making example). In a such case everyone hope also to get the destroyed version of the lab (the one playable in the last level of the story mode against the final boss).