I would guess that "they" are "the capitalists" as a class. It's very common to use personal pronouns for such abstract entities, and describe them in behaving in a goal-driven matter. It doesn't really matter who "they" are as individuals (or even if they are individuals).
More accurate would be something like "reducing labor costs increases return on capital investment, so labor costs will be reduced in a system where economy organizes to maximize return on capital investment". But our language/vocabulary isn't great at describing processes.
More accurate would be something like "reducing labor costs increases return on capital investment, so labor costs will be reduced in a system where economy organizes to maximize return on capital investment". But our language/vocabulary isn't great at describing processes.