What's the joke here? What's a "fed"? Is it a group of people that the middle blob doesn't like and substitutes "Nazis" with that word, which is what he complained about and that's the joke?
A 'Fed' is slang for a member of the enforcement agencies of the Federal government. In other words, the middle wug is saying "They're not actually a Nazi, they're an undercover cop to make 'us' look bad", the joke being both the hypocrisy of accusing someone without evidence, and the impossibility of proving the far-right credentials of cretins to people who are determined to never see Nazis no matter what do to.
I'm with you. This is joke relies very heavily on a stereotype that if you aren't aware of, makes almost no sense. The logical leap between Panels 3 & 4 demands serious background knowledge.
I guess you can still laugh at the absurdity of a person unwilling to call someone a Nazi