Smoot Hawley passed the house in May 1929, about a month before the crash. I think the author is saying that the market may have been reacting to the fear of its effects once they saw it pass the house. It wasn’t signed until June 1930.
The crash was in October ‘29 . The act just made a bad situation worse.
See the late Jude Wanniski’s book, The Way the World Works for the relationship between progres toward passage of Smoot Hawley and the 1929 Crash.