Sadly, sitting 30mph under the speed limit is also illegal. As is impeding the progress of other motorists. And causing an intentional blockage.
So which is the lesser of two evils? Being so unaware of your surroundings, the law and your vehicles abilities (regardless of driver abilities), or overtaking on the wrong side to keep everything flowing?
But he's right, there is no such thing as "the fast lane". If you are not overtaking you should ALWAYS be in the very left hand lane. Funny how people don't like it when I move from the left lane to the very right lane in one go, pass the car in the middle lane, and then go right back across in front of them to the left lane. Some people get the hint. Most don't even notice a car just overtook them at all.