So, we now know that all Alternate Universes/Realities/Timelines are separate from each other in every way.
Different Shadowlands, different Twisting Nethers, different versions of characters, etc etc.
So the AU version of Gul’dan somehow gets sucked into the Dark Portal, right? Archimonde yeets him through because he made a Pact with the Burning Legion.
Yet … Gul’dan ends up in our Timeline, and our Legion is the one pulling his strings.
The working theory I and a couple of other meme lords chronic 
postersgrass-phobic individuals nerds have come up with is that, because Garrosh invaded from our Timeline, our reality, things from our reality could reach into that one, and due to Kil’jaden having turned away from Draenor after attempting to unleash the Orcs on the Draenei and knowing, one way or another, his agents on this backwards, savage world would get the job done, moved on to more pressing tasks, the AU Legion had ceased paying attention to Draenor directly.
And that’s where our Archimonde pops up and pretends to be ‘their’ Archimonde. None of the Legion are willing to question why The Defiler’s energy seems a little off because Archimonde is not that kind of boss to tolerate back-talk, so they’re just going along with it, and that’s also why the Fel Orcs of Draenor looked so strange.
We know that the Furies of Draenor didn’t quite work well with Thrall (and presumably all Shamans as a result) because we were from a different reality and didn’t quite ‘belong’. The intent was there, the communication worked to a point, but it was like both parties were speaking a different dialect of the same language. Likewise, when MU Archimonde gave Gul’dan a heap of Fel-Blood to create the Fel Horde, this was the Fel from a different timeline, an alternate reality. The conflicting energies of the Orcs lingering spiritual essence, the Fel magic Gul’dan was using and the alternate Fel he was getting from Archimonde created true monsters, things of two different versions of time combined into one.
If this theory holds true, remember that at the climax of the Draenor Expansion, Archimonde freakin’ dies. He takes us into the Twisting Nether as a desperation move to kill us because we’re just tearing him a new tail-hole for a second time, and he’s getting Mount Hyjal flashbacks. He needs to divide and conquer and surely in his native realm, where the Fel flows freely, he will be that much more powerful? But it all ends in disaster anyways.
So the thing that bakes my noodle?
Did Gul’dan summon our Archimonde, meaning that once the Black Portal prolapsed and the timelines drifted apart, the AU Legion had their Archimonde and the soul of our Archimonde being sucked back to Argus to be revived and now we have Archimonde in stereo, or was that their Archimonde who got called through trying to figure out who was using his name in vain and got summoned into the middle of a curb-stomping with no idea of what he was about to face.
While I’m more than grateful for the whole “No your souls don’t fuse together with their AU counterparts after death and every timeline is distinct and does not share planes, individuals or similar”, this is kind of a major plot-hole that kind of leaves Legion in the lurch.
Or was it just that because the Black Gate had been used to breach between realities, a more likely situation is that Gul’dan accidentally got yeeted into our timeline due to the blending going on as our two timelines were overlapping artificially due to the Hourglass, and our Legion, never wasteful, decided to grab this knock-off version of the Grand Warlock and try to use him as a figurehead again?
