Know… what? I pointed out a number of things to you; not just one.
Claim 1: What was discovered outside of Darnassus by Jaina was a portal to Dalaran; not a portal from it. That's why the Sunreavers were no longer there; they used it to go to Dalaran. Whenever you see a portal in-game, you can use it to go to the portal’s specified location, but once you’re there, you can’t simply jump back into the portal, because there’s no portal to jump back into. You’ve been dropped off. If you want to go back, the only option to do so is to create another portal. I know this because I’ve played a mage as my main for years. I’ve opened portals. I’ve teleported. I know this from experience. I suppose, though, that even if portals were both ways, it still wouldn't mean they "used Dalaran," only that's where they happen to come from, just as I could stand in the middle of the Violet Citadel in Dalaran, open a portal to Stonard, and kill Alliance attacking it without being accused of "using Dalaran" to carry out my actions against the Alliance; simply because I traveled from Dalaran doesn't mean I used Dalaran to accomplish my ends.
Claim 2: The portal to Dalaran was outside the city. I know this because I participated in taking the Divine Bell. I saw the Sunreaver camp, enveloped by an invisibility shield, outside Darnasuss – not in it. It was that camp that Jaina’s blue orbs led her to in the Alliance quest. If the portal was outside the city, then it could not have been the already existing infrastructure connecting Dalaran to Darnassus; they weren’t even in Darnassus.
Claim 3: Mages can open portals to Dalaran anywhere in the world. How do I know this? I’ve done it. I can hop on my mage and go into the very heart of the Black Temple or the depths of Deepholme and I can create a portal to Dalaran anywhere I please. This does not, however, mean that I was “using Dalaran” in whatever I was doing in these places.
Additional Claim: Mages can open portals to a lot of places; not just pre-set faction cities. This is evidenced by Rhonan who, just as the mana bomb was about to destroy Theramore, opened a portal leading a fair distance outside the city, and pushed Jaina through it. He didn’t need to rely upon some sort of pre-existing infrastructure (the portal led to the wilderness along the coast, not some high traffic area people traveled often). If Rhonan can quickly open a portal by himself to some place in the wilderness clearly outside a faction city without “using Dalaran,” then why in the world would other members of the Kirin Tor (like the Sunreavers) be unable to do the same? There’s simply no reason to assume they couldn’t and history shows us that they could, contrary to your claim.