And it looks to me like we are mapping each row with a constant y, calculating the "distance" (thus scale factor) only once using just the vertical slope for the row.
For the record, there were constant-Z full 3D engines in the late 90s, which would find the correct axes on screen to render perspective-correct textured triangles using oblique spans of pixels. They were incredibly complicated and prone to holes at the slightest numerical accuracy, but they were a sight to behold. Constant-Z meant not just saving on perspective divides, but also easy Z-buffer and depth-based fog.