I can confirm this. I just tested it out in a game. It is literally just as you're passing into the fog, right on that line, if you move into the fog just as you hit them with wither, it will go on CD but they will not be slowed.
Myself and Yamamoto went into a game as opposing Nasus, and if I flashed over a wall just as I withered him, or if I was walking around a corner and turn to wither him just as I do, I'll cast, spend mana, go on CD, but he will not be effected at all by Wither. We attempted this near Dragon, in the bushes, and around that bend when heading up to Golem.
Update for Picture:
You can see the Nasus selected (Yamamoto, with only boots) has no wither effecting him, but mine is on CD.