Not everyone is fully utilised at work, nor does every job's output scale linearly with incremental effort. So while the above is true in some cases (particularly for 'assembly line' type jobs, including programming, where there is a constant supply of discrete and valuable tasks) it is also the case that many people are productive for far less than the full time they spend at work and that a shorter working week would result in significant & near immediate productivity increases in a good number of cases. Examples would be bureaucratic work, a lot of generic 'project' roles, and situations where procrastination is common.

Perhaps it's not that people are particularly much more productive over short hours as that presenteeism and the need to be seen to be working long hours masks the true amount of productive time and many people will do less than they could if they think they can get away with it (particularly if they work long hours at a job they dislike), nonetheless it would be better for unproductive time at work to be spent elsewhere and reducing the length of the work week could achieve this.