What he's saying in the video that you linked isn't that Zygarde is (part of) the original Tao Dragon, but rather that Zygarde split the Tao Dragon apart in order to protect the world from its colossal power, because that thing is easily at Mega Rayquaza power levels.
I don't really agree with most of what Tyranitar Tube said though. Yes, there is some link between Unova and Kalos (the fact that X/Y and B2W2 take place at the same time, the Zekrom and Reshiram statues in the Parfum Palace garden) but the war referenced in Unova history was a war to unite the various tribes in the region and then the subsequent centuries of fighting between the descendants of the twin brothers, and that took place solely on the Unova continent. We don't even know about the geographic relationships between all the currently known regions. All we know at this point is that Hoenn is near the planet's equator, Sinnoh is somewhere to the north, and Kanto and Johto are on the same continent and are neighbors. It IS possible that the two geographically border each other (neither is an island region like Hoenn/Sinnoh is), but there's nothing to suggest that they are in fact right next to each other, and even if they are, that doesn't necessarily mean that they went to war either.
There's also the question of Zygarde splitting the Tao Dragon into 3 parts. Why exactly would it have this power? He calls Zygarde "the guardian of the world", but its Pokedex entries specifically talk about the ecosystem, which to me is a little different from "the world". I really kinda doubt that Zygarde was what split it into 3 dragons. The two brothers initially worked together to unite the Unova region, and it was only after its unification that they began to have disagreements and fight over them. I take it that the Tao Dragon very much wanted to help both brothers and didn't want to pick a side, but it was physically impossible to support both sides and so it felt that the only option was to divide itself into pieces that embodied the thing each brother valued the most, with Kyurem being a sort of natural leftovers or was there in the event that it could become whole once again. The brothers did eventually decide to stop, but I assume that this was much later in their lives and at this point the kingdom was in the hands of their children/grandchildren, who did hold on to the same values as the brothers but did not decide to stop fighting over them, and so Zekrom and Reshiram had no choice but to support these descendants who reminded them of the brothers they once fought for for centuries until the cataclysmic battle that caused them both to go dormant until present day.
But I'm glad he brought up the God Stone. I've found it very interesting ever since I first learned about it and have been very curious as to what Game Freak intended for it to be/do. I do agree with him that the most likely explanation is that it can be used to (temporarily) revert Kyurem back into the original Tao Dragon.
But as much as I want to see the original Tao Dragon in all it's resplendent glory, I find it unlikely that the Pokemon in the mysterious screenshot is it and that we'll see it in this generation.