^Well, proximity isn't everything. The Romulans have also been very insular for most of their known history. From 2160-2266 and from 2311-2364 they were almost completely closed within their borders. And we know the Typhon Expanse was named by the UFP no later than 2368, and probably somewhat sooner.
Not to mention that Star Charts's 2-dimensional maps don't always give an accurate sense of the true 3-dimensional distance between things. The Typhon Expanse could be considerably "above" or "below" the plane of the page.
They've been somewhat insular at key points, but even during those key points, they were active in interstellar politics, just keeping it below the radar. And before their first period of isolationism, they had a deeply expansionist ethic, as established in ENT. So I think it probably they would have named an interstellar expanse so close to their own boundaries.