Bear with me. The Dark Portal links the two worlds of Draenor and Azeroth, but because the timestreams of these worlds are completely seperate, the point in time that one arrives in when stepping through the Dark Portal to Outland is and always has been “arbitrary”, so to speak.
So what Garrosh actually does is something like recalibrating the Dark Portal so that it points at a different “place” in time on Draenor. The reason it’s not technically time travel is because relative time between the two worlds isn’t a thing.
Garrosh’s new portal leads into Draenor’s “past” only in the sense that we consider Outland as the “present”, neither is technically relevant outside of the world in question. From our perspective, we’re only stepping from one place to another.
I mean I suppose if a person originated in Outland, exited through the old Dark Portal to Azeroth, then took Garrosh’s New Dark Portal back to Draenor, then that person would have effectively time traveled to the past… y’know, by way of two interdimensional jumps. One might argue that time travel alone would be less interesting.