The geometries involved are odd anyway, starting with whether a horizontal ribbon can block the passage of a starship in 3D space. We might do well to say that the galaxy is full of barriers or their nooks and crannies, bulges and loops at relatively short distances from Earth. Some block passage rimwards, some corewards, some clockwise, some widdershins. None need exactly be "at" the rim or core even if so stated, and usually the eps don't even state that much...
Just as with the "why Genesis?" issue, the audience here knows less than the heroes. Kirk shared his thoughts with Sarek; he shares "hidden" knowledge here with the rest of Starfleet about the true specs of the Great Barrier. It may be much more local than we think, or much less famous, much less often visited. Perhaps just a minor bump on the road to the galactic center, whilst God is actually imprisoned inside the bump rather than inside the center, and lies to His follower about this, in order to make His jailbreak easier.
Timo Saloniemi