Why didn't Scott Walker realize he was talking to a prankster who was only pretending to be David Koch?

Because Walker had never spoken to David Koch in his life.

That fact begins a Weekly Standard article called "The Paranoid Style in Liberal Politics." Toward the end:

As the assaults piled up Charles [Koch] couldn’t help thinking of Schopenhauer’s “Art of Controversy.” The German philosopher had noted that people who can’t win an argument through reason attack their opponent’s motivation. “I thought I was cynical enough,” Charles said. “But that was pretty shocking, to see what we’re up against, or what the country’s up against: to have an element like this.”...

To Charles, the call for bigger government was egalitarianism run amok. Liberals, he thought, fetishized equality of condition at the expense of personal liberty. “They cannot stand that some people are better off than others,” Charles said. “I think part of it fits Mencken’s definition of a Puritan: someone that’s miserable because he knows that someone, somewhere, is enjoying himself. He cannot stand that. And I think they all slept through Economics 101.”