It's probably about the same, it seems like the change is producing larger more robust plants which yield more. It doesn't seem like that should either the size of the rice grain itself or the nutrient content.
I don't think that's a safe assumption at all. In fact unless it's somehow increasing the rate of nutrient intake, doesn't that all but guarantee the nutrients are being used to make the base plant larger rather than going into the rice?
The abstract mentions how it leads to longer roots ("produced longer root systems"), which is probably the explanation for all the properties (it can extract water from deeper soil, and it can extract more nutrients from the soil).
My concern is how many generations of crops can one have in such a field before it needs artificial replenishing in the ways we do not do today (see my longer comment elsewhere on the thread :)).