TL;DR: you can get more done if you work 60 hours instead of 40 hours in a week, but the work done in that extra 20 hours might be a small fraction of what gets done in 20 hours during a 40 hour week. This is self-evidently true for many jobs. E.g., if you pick apples for 40 hours, then even if you only pick one apple per hour after that, you will still pick more apples working 50 hours than 40. We software developer…
In his example of getting research funding, it's a step function: either you get funded (y=1) or you don't (y=0), and because you're competing with others, even a small output increase can take you from the 0 side to the 1 side.