Your explanation is not clear to me. How can you compensate for the effect of pointing tge camera downward by moving the camera only in the x-y plane?
By shifting the sensor. This is reason why the 4/3 standard creator Olympus defined the image circle diameter, not the sensor size! So the sensor can be shifted around so much that the framing can be kept stationary while the handshake the people get (Olympus really has lots of research and studies done for everything from hand shakes, hand sizes, wanted DOF, sharpness, colors all of it!) that it is compensated without vignetting the sensor by clipping image circle by shifting sensor.
You can compensate for changes in the framing but the sensor and therefore the focal plane was tilted by your movement.
The IBIS corrects only very small motions, not huge ones that would be required so the focal plane (DOF) would change so much that the subject would become out of focus.
If you use center focus point and you do "focus + recompose" method you can easily throw subject out of focus like in portraiture move the depth of field so much that instead eyes being in focus now their ears are in focus. But that requires far more camera tilting than IBIS can compensate in first place.
The IBIS is not like a rear standard in large format camera. You are not going to do a 30% shifts movements for composition or framing reasons. It is there just to minimize the camera shake and to do so it is controlled by the algorithms that are patented by Olympus that what shifting controls are done for the sensor when specific kind 5-axis motions are on camera body.
Notice that Olympus can't do 6-axis correction, as the 6th axis is the Z-axis movement (backward and forward) and that would allow to change the focus by doing very careful sensor movement forward or backward, and that would be possible if the sensor would be
tilted.
The sensor is floating in magnetic field, locked in specific flange distance but still freely floating when not suspended in the field. It can as well be locked to specific position with magnets so it doesn't move at all in normal operation if you don't want to use IBIS.
So if Olympus would make a 6-axis IBIS, it would change again the market a lot as you could now do a movements for macro photography on sensor instead lens AF.
Pentax has the algorithms for the sensor shifting so that you can do small framing corrections by moving sensor up/down or left/right without starting to move the camera itself or needing a macro slide.
