As it turns out:

In a deleted scene just before Vito, Tom and Sonny discuss the upcoming Solozzo meeting, Vito asks Tom about his meeting with Woltz. After Tom tells Vito that Woltz is "not a Sicilian," Vito says, "Send Luca Brazi to me. I think we're going to find a way to reason with this Mr. Jack Woltz."

It's true that the novel doesn't identify Luca with the horse-killing. And, the deleted scene doesn't prove that Luca actually killed the horse--perhaps he organized it, but didn't wield the fatal axe. But, I can't blame Harlan Lebo for concluding that Luca did it.