I've seen the ad before. It is really well done.
1) You are told to count the passes for the people in white so you concentrate on the white shirts rather than the people. I know I couldn't tell you sex or race - only that they wore white shirts.
2) The monkey suit was black.
3) You were focusing on passes rather than people.
4) I wonder if the monkey suit would have been noticed more often if you were to count the number of passes for the people in black?
It's supposed to trick you. People don't like being tricked.
I can tell you that I've never had a cyclist sneak into my vision who was already ahead of me and I'm very diligent about using my mirrors to watch for automobile traffic behind me but I have missed a few bicycles that have sneaked up from behind while maneuvering through congested traffic. Haven't killed any of them yet...but thought about it.