"Spectacularly wrong" here appears to mean that he correctly identified that the bluestones came from the Preseli hills almost 200 miles from Stonehenge (and was the first to do so), but didn't get quite the right outcrop.
He thought they came from Carn Meini; the article says we now think they came from Craig Rhos-y-felin and Carn Goedog.
https://www.theguardian.com/culture/2013/nov/20/archaeologis... says that Carn Goedog is less than a mile from Carn Meini.