Something no one has mentioned, is that the expanse may be "above" or "below" us. The galactic plane is 10000 light years thick, so it could easily be above or below us, especially as star charts and stuff have placed stuff in all deirections in our plane, but hardly anything out of it.
Recently I have had thoughts that the Expanse may actually be another name for all of the different versions of the "Center of the Galaxy" which we've seen over the years. Perhaps it fits, perhaps it doesn't, maybe some of the expanse did remain after the events of Enterprise's third season. There have been several inappropriate and incorrect references to trek and the center of the galaxy, the only one that fits is really from TNG's Nth Degree.
Perhaps parts of the Expanse did remain and somehow the name "Center of the Galaxy" was applied to it for some reason. As another poster suggested if you take into account the 10,000 light year thickness of the plane of the galaxy....perhaps they are referencing that.
Just rambling, and trying to put two ideas together.....