I think some people here may be assuming that "isometric" means to isolate a single muscle group, which it does not. Isometric drills are any static posture that requires opposing forces to maintain, such as the iron cross, L-sit, or holding a partial squat.
This definition is incomplete, and may explain why you're confused. As the exercises you cite suggest, isometric drills are for the purpose of developing muscular strength. IS drills aren't.
It's actually an interesting thought experiment to think how something like the iron cross could be turned into an IS drill, because it might illustrate the differences. So you might be told to hold the position but think about the strain being taken not by the lats but moving it around to different parts of the body. Maybe you'd visualize the hands connected by bungee cords to the small of the back. You'd certainly be told to relax (!) and to be able to move freely even though you're suspended in the air.
The exercise is silly, but points out some of the differences. E.g.: the strain's in the wrong direction, so the visualization doesn't really make sense. There are no alternative deep muscles (so far as I'm aware) to take the strain. And it's not whole-body because the legs aren't in the picture.
But that's what you're talking about --this thing that is ridiculous and silly is equivalent to what the IS people or doing. It's not.
It's still a duck.