I thought the post above with the following comment explained it:

Stewards George Slender, Scott Chaney and Grant Baker studied films of the incident for several minutes before ruling there would be no change.

"In our opinion, the tapes could not verify that the horse was actually hit," said Slender of the unanimous ruling.

I guess they missed it then. I would need a closer look also. They also stated that it had to be a willful smack?