But The Magicks of Megas-Tu doesn't leave any room for arguing about where they're supposed to be. Kirk logs his mission as one to get to the center of the galaxy where matter is being created. They get to a place Sulu identifies as the galactic center. They observe matter being created. They get caught in the space weather and aim for the space center of the space storm.
Ah, but that's the very point of the fun. All this is debatable, as the wording isn't the one you suggest here. Kirk isn't tasked with "getting" anywhere, he's to "investigate". They then see a supposedly vast phenomenon on the viewer, and Kirk is satisfied that the investigation can begin, without any explicit "we are here" comments.
What connects the space storm to the center of the galaxy? Very little. It's identified as "that thing", and there's plenty to choose in the chaotic view. Indeed, Spock further identifies it as "some form of matter-energy whirlwind" rather than galaxy center or anything so far associated with galaxy center, reinforcing the idea that it is a separate phenomenon.
There's no a priori reason that "the point where matter is being created" would have a null center, so that's not a connection. And yet they do reach "the center of things" and drive through. "The center of everything" is big words for a local phenomenon in the middle of a humdrum little galaxy - meaning it's scalable up and down to pretty much every value of "everything". If there's something metaphysical making the center of the storm the center of "everything", this places few requirements on the positioning of the storm...
Spock works out that they travelled through the galaxy creation point
It's lucky that he doesn't say it in as many words, then...
The most you can try selling is that they're actually one sensor-range distance away from the dead center of the galaxy. But that distance isn't enough to appear on a map of the galaxy, and it's not like the last one-sensor-range-distance would be the one that takes all the time to traverse.
Sensor range depends on what is being sensed, even in Trek. Might well be tens of thousands of lightyears for something as big as the center of the galaxy!
Remarkably, "Magicks" in the end is an episode where very little travel explicitly takes place. Any implicit travel depends on a series of assumptions, and those of the writer aren't the only ones that can be made.
At this point, trying to say they're not at the center of the galaxy is like trying to say the wormhole in The Motion Picture was a freak coincidence and not related to the experimental warp drive.
Well, since the 2009 movie establishes that freak coincidences define Kirk's life...
Timo Saloniemi