I personally see no validity in your statement.
Would he have known? Alert drivers are sometimes aware of their surroundings, and usually know where other cars are in relationship to themselves. I like to think I am an alert driver and know where others are but always double check my mirrors or look over my shoulder to see if there are other cars around me or where I might be merging.
Might another car have rapidly approached him in the middle lane, or perhaps changed lanes and ended up next to him? If it was a small car and the driver was in a truck, might he not have seen the other car?
Yes, these are hypothetical questions and yes there was nobody there, AND yes, I agree I would rather sideswipe a car then get the back of my face punched by a semi.
Still, what makes you think he absolutely would have known if there was someone there?