Marx wasn't really advocating that there is a fixed pie which needs to be forcibly carved up. Mostly he built his ideas around the distinction between different kinds of "value", particularly the labour value and exchange value of goods.
His clever insight, the basis of his theory of exploitation, was that labour value and labour power are different. The labourer sells labour power. The difference between the labour value of the good and the labour power sold accrues to the capitalist.
One of the several ways he was wrong was that prices aren't really set by labour value at all. All economic value is subjective, influenced by many factors. The average amount of labour is neither here nor there.
His clever insight, the basis of his theory of exploitation, was that labour value and labour power are different. The labourer sells labour power. The difference between the labour value of the good and the labour power sold accrues to the capitalist.
One of the several ways he was wrong was that prices aren't really set by labour value at all. All economic value is subjective, influenced by many factors. The average amount of labour is neither here nor there.