I've been thinking of this, especially in light with the tragedy in Christchurch today. I think this view is fundamentally mistaken in two important ways. The first is that as I said, when we pretend to be something we become it. When we argue for a position, even as a hypothetical or a joke, we start to empathize with it more. I think a lot of people start out joking about something and gradually begin to seriously believe it.
The second reason it is fundamentally mistaken is that it ignores the fact that there has always been a "trolling" aspect to far right discourse such as Nazism. Think of the racist carictures Nazi's or KKK members draw, they are meant to be taken "seriously not literally" as some have said of Trump. Reactionary politics goes hand in hand with intentionally trying to provoke a reaction, both of them have this shared idea of trying to gain power through provocation and the idea that the status quo is a sham. As Sartre said in Anti-Semite and the Jew:
"Never believe that anti‐Semites are completely unaware of the absurdity of their replies. They know that their remarks are frivolous, open to challenge. But they are amusing themselves, for it is their adversary who is obliged to use words responsibly since he believes in words. The anti‐Semites have the right to play. They even like to play with discourse for, by giving ridiculous reasons, they discredit the seriousness of their interlocutors. They delight in acting in bad faith since they seek not to persuade by sound argument but to intimidate and disconcert. If you press them too closely, they will abruptly fall silent, loftily indicating by some phrase that the time for argument has passed."