Oh, certainly the archaeology proves full well that Stonehenge was around quite some time, by the time of Hellenic contact with the Brythonic tribes of Great Britain; but to be fair, this association goes back to Diodoros Siculus', Library of History [there are quotes on the Theoi.com page for Hyperborea], and he mentions "an island across the sea, north of the land of the Keltoi, home of the Hyperboreans"; there were certainly Keltoi on Breten and Eire [the islands of Great Britain and Ireland, respectively], but linguistically and culturally, they were only broadly related to the Continental Keltoi on Gaul and Iberia [France and the Spanish/Portuguese peninsula], and the Keltoi Diodoros Siculus refers to would be the Gauls and Iberians, and the Hyperboreans would be the Brythonic and Gaelic tribes (and possibly the Pictish tribes, as well — depending on whether or not archaeologists and anthropologists can finally determine if or not the Picts were Brythonic Keltoi or another linguistic and cultural group — like how the Basques are their own thing, unrelated culturally or linguistically to either Spain or France).
…but anyhoodle, certainly Stonehenge existed prior to regular Hellenic contact with the Brythonic people, but considering the compelling evidence suggesting it was initially designed as an astrology calendar and de-facto temple, it wouldn't surprise me at all that later the god/s worshipped there became syncretised with Apollon. The findings suggesting that it was regarded as a place of healing is new to me, but I also knew about the location where the stones were moved from was regarded as a healing place, with healing waters, so I can't say why I didn't put it together, before.
The identification of the presumned-legendary Hyperborea with the British Isles also pre-dates Diodoros, with Hecataeus of Abdera (whose identification pre-dates Diodoros' by three-and-a-half centuries, but today exists only as quoted by Diodoros), and this may shed some light on the presumed-legendary offerings of gifts from the Hyperboreans to Apollonian temples on Delos and in Delphi, packed in straw and passed down from Dodona in Northwestern Hellas. If the assumption is that these gifts did not originate in Dodona, but in Britain (which is identified as Hyperborea by the ancients), then clearly even the Brythonic tribes worshipping at Stonehenge considered their god one-in-the-same with Apollon.
I'd also say, as one who once lived in England, when compared to places where winters are even a little harsher, there should be something to the description of the Hyperborean lands as being of "eternal springtime" and an association with Britain. Being of maritime climate, winters seldom skim below the freezing point, and summers are fairly temperate, and it's a common sight in the middle of the summer to see people at the various seaside holiday spots in Britain to sit on the beach fully clothed, the weather clearly not hot enough to make wearing less seem that inviting to most people.
…and to get a little "woo" for a sec, this might shed some light on my draw to worship Apollon most in winter, and feelings of spiritual distance in summer — my family is British, in origin.