Your summary of Marx's argument in these lectures is inaccurate. In these lectures too, Marx advances his known theory of surplus. He does not say that capitalists pay to the workers the full value of the commodities the workers produce, but the full value needed for the workers and their families to survive.
Quote
We must now return to the expression, “value, or price of labour.” We have seen that, in fact, it is only the value of the labouring power, measured by the values of commodities necessary for its maintenance.
Marx argument is the well-known one: when you work $H$ hours, you produce $Y$ goods that are more than you need to survive -but you are paid only $W<Y$ goods, where $W$ is the goods that you need in order to survive. The rest is the surplus, which due to the socioeconomic structure is kept by the capitalist (and maybe others like landlords), but it should be given to labor because it was labor that produced it, since "capital" does not possess this "surplus-creating property".
Marx's picture is not "you work 8 hours but by abusing as an individual my power over you I only pay you for 4 hours" but "your wage is determined, by the socioeconomic system, as what you need to survive, not as the value you produce. So I pay you what the system dictates, I am not "exploiting" you in the traditional sense of the word. Still, because you produce more than what you need to survive, I can sell the products at their "real/fair" price (so I do not exploit the consumers either) and still see a surplus -which I keep, because the system is what it is".
