It's certainly a theory that's been posited many times by many respected historians, so it's possible.

The problem for things like this is that because almost no one bothered to write about things so banal as which side of the road they travelled on, it's difficult to find any contemporary sources for what they did and why.

The problem with the 'Knight' idea is that it really serves to give little practical advantage. Your sword would only be on the correct side of your enemy was travelling in the opposite direction on their left, in which case their sword hand is on the correct side too. Unlike direction of spiral staircases, for example which were designed to give the defender a tactical advantage over those lower down.

Certainly the idea that Napoleon instructed his armies to travel on the right, which the British begrudgingly admitted was a good idea, but instructed their troops to travel on the left, is likely untrue. Sticking to one side of the road improves traffic speed and reduces rutting and wearing on unpaved roads, it's unlikely no one thought of driving on one side before Napoleon.

The earliest evidence we have for driving on the left is from Roman roads, where deep ruts would wear into the stones. Archaeologists noticed at a Roman quarry in England that the ruts on the left were more worn than those on the right. Presuming that carts left the quarry full and returned empty, this would suggest they preferred to drive on the left.

I know it's a blog, but this site has some fairly well sourced material on driving sides: http://brianlucas.ca/roadside/