Where does it say the A is alpha and antialias value? To me that doesn't make sense (especially given this is an old game you're dealing with).

If it were just the alpha then that would make sense. i.e. a value of 0 means transparent, a value of 15 means totally visible, inbetween varies the opacity of the pixel (e.g. a value of 3 would be quite seethrough).

In the DrawAntiAliasPixel method the X and Y values are the X and Y values of the pixel (where you want to draw it). They are declared as a double since his function performs antialiasing (the process of 'faking' drawing a pixel that's inbetween display pixels) and so can handle pixels at positions such as (12.5, 53.8).

Having an antialias value per-pixel (as you describe) doesn't make any sense. Antialiasing is performed on all pixels of an image in the same way, not specifically to each pixel).