They are specifically pointing out the "blue zones" where a high percentage of supercentenarians live. It's possible that the average literacy of Sardinia is separate from these pockets.
I mean I'm guessing the authors didn't just sit around and say, "hmm, I wonder what the literacy rates are in Sardinia? Let's say low." I guess it's possible though.
For Sardinia they only claim poverty (with a qualitative, non quantitative claim) and life expectancy which, if I'm reading the ISTAT tables correctly, they got spectacularly wrong.
Also there are no blue zones within Sardinia. The demographic data is the for the all the provinces of the island.