The motion caused the Sam to shift back off the line. He did press Fasano from the inside, but his jam is pretty lame. Fasano did jab step around him and then took the post route back to the inside. The ball wasn't thrown then. Whitner comes up at that point, probably expecting what you described, but Fasano takes a good 5 or 7 strides to get on level with Whitner, at which point Whitner is beaten and can't make a play on the ball or Fasano as the ball is dropped in and Fasano catches it in the endzone for the score. There was no switch, so the Sam was playing the 5th in front of Fasano. The Will actually does get a good jam on the flanker on the original weak side.
First, I'd say the Dophins were expecting this vanilla zone defense. The motion just declares the defense and then they ran the skinny route right across Whitner's face, baiting him to try to jump the route, but knowing there will be no real pressure and going over the top. In a cover 2, the under zones aren't going to drop 20 yards deep to help Whitner out. Finally, the Dolphins were clearly wanting to match-up Fasano against Whitner on the play; Fasano has very good speed and a size advantage on Whitner, so even if Whitner had been in position, the play had a good chance. It all worked perfectly for them.