But he wasn't a brilliant general. He was a complete moron completely oblivious to issues like overextension or supply lines. He was fighting the first war in his life for god's sake. Him winning anything significant would be inconsistent with his portrayal. Also, it's not like the Horde achieved any real victory over the Alliance either because it's an MMO and status quo is to be preserved, so your MUH HORDE BIAS doomsaying is rather misplaced.
And what way is it, exactly? The way of simply bending over when Alliance declares war of them instead of fighting back?
Vol'jin failed, not the entire rebellion. Stark majority of the rebellion just landed ashore. With Forsaken and Blood Elves often having some kind of ballistae or catapulsts on their ships. Then there's the kind of siege weaponry you build on the spot like siege ladders, siege towers or battering rams (could even use Vol'jin's destroyed siege weapons for that).
And Wrathion isn't exactly an omniscient being that's always correct. On the contrary, his track record shows he's a complete moron. You're talking about a guy whose brilliant plan to stop the Legion only weakened Azerothian forces by pointless conflict with the Iron Horde AND brought the Legion here. Malfurion and Velen weren't present in the Underhold when Varian made his decision and consequently the Horde leaders outpowered the Alliance ones.
As such he made a different decision and pursued further conflict with the Horde the Alliance leadership present would get wiped out and the main part of Alliance army that was above ground in Orgrimmar would become headless and turn into easy picking for the Horde (especially with Sylvanas already using her Val'kyr to resurrect the humans and bolstering the Horde forces even further).
And what would the crumbling remains of the Alliance do after that? Scream their battlecry of "have mercy"? Flock around Malfurion only for him to run away from Sylvanas like a little bitch multiple times just like he did in A Good War (if he didn't outright break down after Tyrande dying)?
Except the gunship was called in to finish the job when the Alliance was still thinking they could win this fight. Also, the Horde's retreat didn't expose any flank because the Legion forces from Horde's side needed to take a hike around the entire island to reach the Alliance due to the topography of the terrain. Furthermore, Alliance retreated when they did only because they heard the Horde's horn and realized the Horde retreated. And had they not retreated when they did, they'd still be on the ground when Gul'dan dropped his Fel Reaver. Which means the Alliance would get roasted. So the claim that Sylvanas didn't save the Alliance is flat out false.