You have misunderstood the section you have quoted. Paraphrasing, it states: "If a relation has sections with a different route for forward and reverse, then here are some things to be careful about". It does NOT mean "If a relation has two possible routes for a section, then tag them with directionality".

I do not know what problem you are trying to solve by adding directionality, but I can say that having maintained the Fife Coastal Path relation for the past few years, I have had no problem keeping it correct. Please do not add directionality where it does not exist on the ground.