Tom didn't tell Woltz who his client was when he approached him on the movie set. Woltz only learned about Vito after one of his associates checked Tom out. So, it's not totally implausible that Woltz first thought Tom might have been a two-bit hustler from Johnny.
Tom also wondered why Woltz would invite him for a tour of his estate and dinner if he knew he was going to turn down the request to give Johnny the part. Once again, the novel provides the key detail, when Woltz says, at dinner:
"I'm sorry, the answer is still no. But since you're here, what will it cost me to have that labor problem cleared up. In cash. Right now?"
Did he really think he could keep Vito off his case simply by telling that he was tougher than a guy who had a gun put to his head with the promise that his brains or his signature would be on a contract? If so, I think Woltz' brain was going soft from all that comedy he was laying with that young girl.
The novel has Woltz telling Tom that J. Edgar Hoover is a personal friend of his. What was really dumb was that Woltz thought his friendship with the FBI chief would scare off Vito. After he wakes up with the horse's head, Woltz finally realizes, "What was the penalty for killing a horse in California?"
