Marx's writings around the working day are built on the assumption that wages will be set at the level necessary for the worker to survive long enough to turn up to work the following day rather than according to the perceived value of their labour time by the highest bidder, and that because of this near-universal day rate for undifferentiated "labour power", employers' main route to improving profitability would be extending the working day, with no meaningful proportion of the workforce having the ability to change this arrangement, not because they might dislike the status impacts of having to choose a different position, industry, location or smaller house/car/pension but because no capitalist will be willing to pay them enough to eat for a shorter working day. He even devotes a footnote to ridiculing "vulgar economists" for suggesting that labour rates could be set by supply and demand. In this respect, he's far more wrong about future developed economies than Keynes' (later) wrong assumption of a tendency towards a 15 hour week.
Marx's microeconomic theory is even more wobbly than many of his more capitalistic contemporaries at the best of times, but his writings on the lengthening of working days at mills is particularly irrelevant to modern service-based Western societies and even wealthy programmers and lawyers still working more hours a week than they would prefer. There's a kernel of truth in the idea that employers will tend to profit from people working longer than they're contracted to work, but you don't need to read Kapital to understand people like getting more than they originally asked for.