You don't need much supporting evidence for a statement like this when the most common idiom in the American language for an expressionless face is "poker face." Nonetheless, you should watch a championship poker game on television sometime and observe how much emotion the top players in the world display while interacting with one another. You'll see a group of completely straight-faced individuals sitting around a table, slowly picking up the cards that are dealt to them and showing no emotion whatsoever until the game is over. This complete lack of expression and emotion is the logical end of the poker mentality.
Try to imagine what it would be like to do business with somebody who tries to maintain a poker mentality at the bargaining table. You might ask, "So, what are you interested in getting in this deal?" Their response: a blank stare. Then you might try, "Are you excited about our future possibilities?" Their response: a blank stare. Finally, you try to stare back at them, and they respond, "Now we're getting somewhere."
In contrast to the poker model of raising capital, Thomas Edison was constantly enthusiastic about his products and possibilities, and this played no small part in his success in raising capital. He also offered a great upside for his investors and tended to keep a good line of communication flowing between him and them.