I personally do not see where this becomes a confusing thing to understand. Garrosh does not go "back in time" he travels to a different timeline entirely.
It's the concept of in one universe you ate some toast went about your day and everything was fine. In another you accidentally had the toast burned or dropped it and being a few seconds late caused you to get into an accident. In another an armed intruder breaks into your house and shoots you in the head before you even wake up to make your toast. It's the concept that there are million if not billions of different "universes" per say and not a single one of them has jack squat to do with one another.
There will be no saving Draenor in this timeline and having something change about our timeline as other than this bridge that is formed to travel back and forth between the two timelines there is nothing that connects them.