I think the idea is that Garrosh uses the dark portal to go to a draenor parallel to the one in our universe where the demon blood was not consumed. He then redirects the portal there. These are not past versions of the old Orc chieftains, they are them, 30 years older in a world where they did not drink the blood of mannoroth.
It's still dumb as all hell, but it is the only way their claim that it is not time travel can be correct.