Precisely.

Unless they are seriously sugesting that the builders of Stonehenge used every single scrap of glacially deposited Bluestone in Wiltshire (Which is even more outlandish an idea than the Bluestones being transported from Wales), their theory rather relies on deposits of Preseli Bluestone being found in the south west of England, rather in the same manner as the Greywethers.

Have such deposits been found? If so they would also have to be bleedin' obvious or else the builders wouldn't have found them in order to use them.