That was my take exactly! David Mann felt de-manned, as in borderline castrated, by the truck driver. His status was in jeopardy, at home and in work, as we hear in bits and pieces in his phone conversations earlier. Now he's got a madman trucker after him. The guy is rattled, so he asserts what little feeling of power, of superiority, he has left in him, by patronizing the waitress. Interestingly, Jack Nicholson's footloose musician in
Five Easy Pieces
did the same thing the previous year, but he was being an A-H just for the fun of it, while Weaver's character was basically decent, a small potatoes
bourgeois
having a
very
bad day.

Maybe it was misspelt on the menu, and he wanted to be pretentious about knowing the correct spelling. Psychologically, it would make sense: He's trying to recover from this nightmare and get back to normal, so showing off a little superiority would help.