The horizontal timing of the 350-line EGA modes is extremely tight. They say there is no support for overscan in those modes, but with "start horizontal blanking" being bigger than 50 (hex, I assume), there is some overscan. The active period in that mode should be exactly 80 characters, so horizontal display end should be 4F or 50 (I don't remember the convention used here), which is before 52, the horizontal retrace start. So the claim that "there is no usable overscan area", which would go up to "start blanking" is consistent with your observation.