The quote was actually abridged from a bit in Mort. The actual quote is
"Light on Discworld isn't like light elsewhere. It's grown up a bit, it's been around, it doesn't feel the need to rush everywhere. It knows that however fast it goes darkness always gets there first, so it takes it easy."
They left out the good bits to make it seem more sinister, since it was Reid's quote.
"Light knows that no matter how fast it goes darkness always gets there first." is what they used on the show.
I was simply amazed that my husband, a Non-Reader (Gasp) recognized, and said "Isn't that your favorite author?" lol, I guess he does know me a bit, hehehe.
Aha! So, Bob's yer uncle... very clever.