Yes but slaves were never a majority of the population. The majority of the population throughout the Republic and the Empire were wage labourers and self employed.
Marx's definition of a pre-industrial state is basically one where the majority of the population still owned and controlled their own "production". Aside from the slaves (who nonetheless were allowed to keep some of their earnings) the Roman empire belonged to this category.
Strictly speaking if we go by the definition that capitalism is "private ownership of the means of production" then many pre-industrial states would qualify. However, again, people heavily load the term capitalism with other "modern" institutions like corporations, and Marx himself was specifically objecting to the form of capitalism wherein large-scale factories are owned by a handful of individuals while the majority of the people actually working at the factory are wage-earners.
That people - including Marx himself - keep failing to decouple the issue of "private ownership / small business" and "large corporations where most of the workers are wage-earners with no ownership" is a big reason why corporations get away with exploiting their workers so much.