I'm afraid there's a couple terms here I'm not familar with, I'm wondering if Bill Amend made a mistaken assumption that the ball's motion is the arc of a circle, rather than a parabola, as the range formula has an extra term. :-P Usually Amend gets his physics right, and more so than I would've thought of off the top of my head, so I'm suspecting that the mistake is mine.
ETA: Jethereal and I haven't gone through the math, but we're pretty sure what he did was assume the ball was travelling in the arc of a circle while Jason was holding it, hence the d, l, and α, and then when he lets go it travels in a parabola with the normal range formula. You'd want to let go of the snowball at an angle α=45º=π/4 as it would then be launched at the same angle, which gives the best range. The extra term to the range formula is probably for that circle arc, though technically he should also account for his own height. :)
Ah, the wonder of theories! A biologist/teacher I was talking with last week said she really wished scientists had picked a word other than "theory" for it.