The normal recorded form of title of the Scottish kings found in primary sources "rex Scottorum", frequently translated into English as "King of the Scots" rather than "King of Scotland", is of little practical significance as it mirrors the practice in chronicles which describe the king of England as "rex Anglorum" and the duke of Normandy as "dux Normannorum". The form "King of Scotland" is therefore used throughout this document rather than "King of the Scots". https://fmg.ac/Projects/MedLands/SCOTLAND.htm
The term king is a functional translation of a job description. There is insufficient documentation from 8th / 9th century Scotland to say whether it is an accurate description, but the preeminent modern historian of the Scots - Alex Woolf - certainly uses the designation eg Fergus son of Eochaid dies as king of Dal Riata in 781.