It does not draw borders but it does detect red pixels in an effort to guess neighbors (or it did in dom2 and I am pretty sure it does in dom3).
I gave a lot of thought to this when I was working on my own custom map editor. If you draw an imaginary line between two points (provinces) and check each pixel's color along that line and set a flag when you hit a border (red) pixel, then skip ahead a few pixels (to clear a border that might be several pixels wide at that point) and if you reach the 2nd point without hitting another set of red border pixels, then you can say that it is likely for the 2 points to be neighbors. This is not 100% foolproof, but it can generate a fair guesstimate.
