I am thinking that the cover plate is designed for this as well with the brown sliding material on the outside. But it doesn't have the base plate cover. Also I think you could flip the parallel guide upside down and do this. All in all it seems that holding the plunge saw upside down against a ceiling and moving it along while keeping it plunged could be quite the feat!
I think I get it now, although I don't see any shadows in the photo . In addition to back beveling the scribed joint to get a tight fit, you add a front bevel to add an intentional shadow on the cabinet/wall joint so the small gap that opens during the dry season is in shadow throughout the year..
I have a Bepo Twist,which does a similar kind of thing,you set the height and depth,then put it against the skirting to cut a new expansion gap in the floor or on the floor to undercut skirts,i wish this festool attachment was about when i bought it,it would have done the same job for a lot less money