Hi CKMaui,
That’s correct. If you have a 6" core and you cut a 1", a 2" and a 3" layer out of the core they would all be rated at the ILD of the original core although they would all “feel” different because the thickness of a layer will have a significant effect on how it feels and performs compared to the same material that is thicker or thinner (see post #14 here for more about the effects of the thickness of a layer or a mattress).
The actual “feel” of a certain ILD is subjective but in general heavier people will sink in more deeply to a specific layer than a lighter person so the same material in the same thickness may feel firmer or softer to different people.
Phoenix