This latest theory is based on the perception that the Preseli blue stones are what it's all about -and Welsh peasants- even into the modern era- regarded the springs around the blue stones as having medicinal powers.
Tenuous, very tenuous.
But even if we concede that the stones were seen as curative, was this their primary quality, or a by-product of their being holy? In the normal run of events the holiness comes first. Lourdes, for instance, is what it is because a Divine personage once appeared there. The Holy heals, but it heals because it's Holy.
Pilgrims flocked to the shrine of St Thomas Becket from all over Europe: some of them hoped for physical healing- and a very few of them experienced it- but to call Canterbury Cathedral a medieval A & E would be to miss the point, I think.