So this is new to me and I am not sure I fully understand all the physics behind it. All my previous cameras have not been ISO invariant, that is to say to minimise noise you needed to make sure the image was not underexposed by using an appropriately high ISO. Using a lower ISO and underexposing an image and then pushing the shadows in post would create a noiser image than using a higher ISO and “proper” exposure. Pretty standard stuff and well understood.