Actually I think the portal was rebuilt..though it has been awile.

I wouldn't be surprised if the azeroth side was done by Gorefiend and his Deathknights, of all of Warcraft's villains, Teron is probably one of the most cunning and intriguing.

The most interesting thing about that character to me though, is that he both rode Deathwing, and paid a visit to Dalaran(pre-archimonde destroyed Dalaran of coarse), throwing a coin in the eventide.

Considering Teron Gorefiend's final words in Black Temple and the next expansion, I would not be surprised if he made one final appearance...during the Cataclysm pre-release event no less.

I'm not asking about how it was eventually restored; I'm talking about during the period that this article covers. We're told that Kadghar destroyed the Dark Portal, and then we're told that the Orcs are surreptitiously using it to travel to Draenor and back. How did they use a destroyed portal to travel back and forth?

It's because it wasn't completly destroyed, there was still a great deal of energy left after the structure itself was destroyed. It became a round mass of unstable portal energy that was hidden inside the crater that the current portal resides
today, before the initial destruction the crator wasn't quite as deep as now. What Teron stumbled through was the unstable mass before waiting 2 years with ner'zhul

Thank you, Destiny's hand, that explains it.