I think that she is referring to the system of “indentured servants” who would sign up for a period and who were used as white labour in the American colonies in the 17th and 18th centuries.
They were not slaves as such, although their contracts could be bought by colonists from the ships’ captains who transported them across the Atlantic, sometimes for nothing. Often, this was to give them a new chance in the New World by a family that could offer them nothing at home.
After completing their contract – often about five years – they would normally be freed and would be given land.
This enabled European colonists to increase in number at little cost, indeed, at a profit, to their poverty-stricken families at home.
It was here, however, at home in Scotland that the last slaves remained. They were the miners and salters who were serfs and they were not freed until 1775.
However, in criticising the kidnapping of Africans, Ms Roe should also remember that the peoples of Europe, including Scotland, had also been subject to slavers raiding our coasts from Africa for centuries before that. Sauce for the goose, perhaps.
Andrew HN Gray