I am led to believe that other genetic samples from Sardinia don’t necessarily show the same degree of weirdness that you see in the HGDP samples: they look more admixed, more cosmopolitan. The guess is that Cavalli-Sforza knew what he was doing, and chose samples from the most isolated, mountainous, gnarly villages.
In a way, this should encourage people searching for genetic traces of the far past. What we see in those HGDP samples is not just a product of the unique history and ecology of Sardinia (although that surely helped) – it also depended on very local effects. This suggests that you might find interesting genetics in a number of out-of-the-way, hard-to-get-to corners of the world. Places that the tax-collectors couldn’t even reach. Maybe even in Europe, not just in Kafiristan or Shangri-La.