There’s a crafty bit of rationalisation in the Castleden book , initially he says “there was a lowland route connecting Skara Brae with the ceremonial centre on the isthmus “ then in the same paragraph the route becomes “, the road turned east along the southern shore of the Loch of Harray …..” . We can all find routes but a bit more difficult to find an actual road .
He also mentions the Sarsen Road from Avebury to Stonehenge ( in the same chapter ) equally as subjective and with no archaeological evidence to support it at the time of writing (1987).