Keeping left means not allowed in the outside lane unless overtaking. You can still be in any other lane. And in that road the left lane is for slow vehicles, the middle lane where the car was is for normal speeds and outside lane is for overtaking - pretty common all over the country. Driving at 20kph in the centre lane might be considered impeding traffic but driving at 70, 80, 90, up to the speed limit in the centre lane is not impeding traffic - the truck driver was a total douche and should be penalised as such.

