Here are the facts:
Trade only amounted to 6% of GNP to begin with, and it declined to 2% of GNP by 1932. Furthermore, since Smoot-Hawley only applied to 1/3 of imports, you can't even attribute the 4% drop to the tariff. Rather it was the 31% decline in GNP and 25% unemployment rate that more drasticly reduced demand for imports.
A Google search reveals sufficient background information on the two relatively unknown wannabes you cite. However, the mere fact that you reference them in support of your absurd contentions is enough to condemn their credibility. They eschew valid academic methodology by assuming the politically correct result they desire is "true", then manipulating historical data and facts to support their incompetent assertions.
Senator John Heinz III, who died tragically in a plane crash in 1991, had developed a national reputation for his expertise in international commerce. During his years of serving in Congress, Senator Heinz III was appointed to the Chairmanship of the Subcommittee on International Finance and Monetary Policies. He had this to say about the Smoot-Hawley myth in 1985:
“It gravely concerns me that every time someone in this administration or the Congress gives a speech about a more aggressive trade policy, or the need to confront our trading partners with their subsidies, barriers to imports and other unfair practices, others in Congress immediately react with speeches on the return of the Smoot-Hawley Tariff Act of 1930, and the dark days of blatant protectionism and depression...It seems that for many of us that Smoot-Hawley has become a code word for protectionism and, in turn, a code word for the depression. Yet, when one recalls that Smoot-Hawley was not enacted until more than 8 months after the October, 1929 collapse, it is hard to conceive how it could have led to the Great Depression...the changes supposedly wrought by this single bill in 1930 appear fantastic.”
BTW, the late Senator Heinz entered Harvard Business School in 1961, and the following year worked for the summer with the Union Bank of Switzerland in Geneva. He received his Master's degree in Business Administration from Harvard in 1963. That's sufficient academic background for competent understanding of economics to be authoritive on the subject, unlike your voodoo propagandist Wanniski.