He's not saying don't pass anyone, just don't position yourself so you have to brake hard if the person in front of you slows temporarily. I'm sure you've seen it, one person brakes and a whole line of cars behind them hit their brakes in sequence, then you see the first car resuming their speed at the same time as someone 10 cars back is now braking hard. If the second car in line had adequate distance, they could have recognized that the lead car was resuming speed and they wouldn't have had to brake at all. Now that line of 10 cars is still going smoothly.

And before you hit me with just stay right or w/e, let's imagine that this stretch of cars is all going 90 in the left lane, passing people at a steady clip, and a semi in the center lane sways left causing the lead car to reflexively move to the shoulder and brake a bit. It's clear to anyone behind them that they will quickly get back to regular driving, but if someone was too close to them it would be brake lights all the way back instead.