The problem with the half-link chain is that instead of alternating inner and outer plates that are straight, every single side plate has a slight S bend to it, which wants to straighten out under tension. This leads to genuine chain stretch, as well as the traditional elongation of the bushings due to mechanical wear that every chain will experience. There is no such thing as directionality with a bike chain for the purposes of what we are doing with them. Pulling on something, by definition, means there is a force going each way, so who is to say which way is forwards or backwards? I think it's more the case that the chain on a fixie is more likely to be stressed harder than chains used on other styles of bikes du to the rider mashing hard from a standstill and skipping the wheel. This extra force will try to straighten the bends in the side plates, making them each slightly longer than they used to be.