All I get when I click on this link is "Access Denied".
However, just because Wesley said 19% of the galaxy had been explored doesn't necessarily mean (1) it was all by the Federation's assets, i.e they could have acquired information about space "southeast" and "east" of Federation space by the Klingons, and (2) it IS Federation space. There was a line from the DS9 pilot "Emissary" in which Sisko and Dax knew where they had emerged from the Bajoran Wormhole in the Gamma Quadrant because an earlier Federation probe had mapped that area and transmitted its findings back to the Federation.
A few hundred years of warp probes in all directions of the compass and in three dimensions will cover quite a bit of area. In fact, with the current crop of Trek literature incorporating slipstream drive, it's not too farfetched to envision probes studying the entirety of the Milky Way over the next few years, lit-time.