Weren’t those pre-Scottish folk the Picts? I have just finished reading “The Seven Daughters of Eve” which describe how various branches of European origin peoples wandered around and where they ended up.
“The Picts were a tribal confederation of peoples who lived in what is today eastern and northern Scotland during the Late Iron Age and Early Medieval periods. They are thought to have been ethnolinguistically Celtic.”
They were -- "P-Celts", rather than the Scoti who were "Q-Celts", IOW, more closely related to the Welsh and Cornish.