See, this is why I kept calling your lists arbitrary.
Varian couldn't "not" do it, he just
didn't do it. There's a big difference between trying and failing and simply not trying something, and there's also a lot more that goes into prowess as a warrior than a successful coup. Some of the greatest individual fighters in fantasy history saw their plans fail, despite being "stronger" than their opponents or peers.
Kargath also wasn't alone. He led a slave uprising yes, and he was the greatest gladiator slave of his group (so was Varian), but you can't give him sole credit. He also didn't "defeat" Magtheridon, per se. He led a revolt against the pitlord which enslaved it, but it was his entire fel orc clan which did this, not he alone. Likewise, Kargath did not defeat the Arakkoa empire alone by any means. He used the Outcasts to destroy their super weapon (which was decimating his people) and then turned on his allies. By that logic you could just as easily say that Varian "defeated Garrosh", simply because he was the Alliance leader of the assault on Orgrimmar.
How can you rationally diminish Varian's victory over Onyxia because he wasn't alone, and then promote Kargath's defeated of Magtheridon when he wasn't alone either, nor is there any record of events period, in the same sentence?