He did not. He didn't win thanks to a fluke, and the things that defeated him were the crystal ponies by powering the crystal heart. The main six and Sombra never interacted with each other, so what the hell are you talking about?
Did Hasbro gave us a reason behind Discord's and Chrysalis' incompetence? If the reason was that they were overly confident, that was executed HORRIBLY. They didn't come across as overly confident as much as simply negligent and stupid considering the situation. I don't care about deep character development if incompetent villains is the end result we're getting. No character development in the world can make up for what happened. Discord and Chrysalis are broken beyond repair.
At least Sombra has an excuse for his defeat - he doesn't have wings and there wan't enough time to go anywhere or do anything from where he was standing. Nightmare Moon, Discord and Queen Swisscheeselegs on the other hand - their enemies were right there in front of them. They could have gotten rid of their troubles so easily, especially Discord. But nope, since incompetence was a thing for pre-Season 3 villains, they just stood there allowing their enemies to take all the time they need, do everything they need to do to defeat them.
There's a strong theory that Sombra is the creator of the Alicorn Amulet, and since alicorn magic is revealed to be dark magic, implying that dark magic is more powerful, this makes King Sombra the ultimate user of dark/alicorn magic. Sombra is like the Yin to Celestia's Yang, he's the anti-Celestia. Those are some pretty unique things for an MLP villain.