> By an astonishing coincidence, the number of bits in the input format is approximately the log2 of the number of MD5 evaluations that a five-year-old GPU can do in an hour.

I read this to mean he calculated a matching hash on his GPU, in order to tune the magic constants of his app to produce the desired output.

> There are no magic numbers in the program

There are several:

    long z,x,G;main(){for(puts("P1\n80 80"),scanf("%10lx",&G);3-z/2160;x=++z%81/8-5)putchar(5>x?!(16>>(x^-(x<1))+1&G<<5>>z/648*5)^49:10);}