It's not paradoxical at the extremes, but rather in how it goes against the beliefs that many people put into practice. 'I need to go in to the office this weekend because I have a lot to get done.'--'I had to pull an all-nighter to get the project done.'--'I stay late a few nights a week to make sure I get all my work done.'

Incidentally, in all three of those examples I suspect the advantage comes from working when fewer people are available to distract you, but the implication is always that you get more work done by spending more time doing it, which is being refuted as a valid concept for knowledge work.