What constitutes the slurry of a J-Nat? The answer is slurry particles, or plates. If you want to call them something else that is fine, the name is irrelevant. What matters is how are they composed and the pictures show they have layered structure, like puff pastry. That would indicate that break down happens through shearing predominantly but the way to prove or disprove is to measure the thickness of the plates and that has not been done to my knowledge. I have worked with an SEM quite a bit and am sure that thickness measurement can't be done with that instrument.
In Japan it is accepted fact that the slurry breaks down, since those guys have used those types of stones for thousands of years, I think they probably know what happens to the slurry.