Cantor wasn't kidding. The reason he seems so knowledgeable about the stock market in this bit is because the Crash of '29 left him deep in debt. 

How do I know this? In Groucho Marx's autobiography, "Groucho and Me", it was Cantor who convinced Groucho to buy stock in Goldman-Sachs (yes, it existed back then), to which Groucho replied, "What is Goldman-Sachs? Is it a flour bag?"

Cantor was a little too persuasive and Groucho bought $35,000 of it on a 25% margin. In the end, he ended up losing $240,000 in the market in total - or so he claims - it was likely a lot more. At the end of the chapter, he said Cantor's advice was the worst of the lot, "Eddie, how could you?" he writes.

Groucho said some of his friends lost millions in the crash. He was talking about Eddie Cantor, among others. So the lesson here is 'never take financial advice from comedians'...or something.