Mr. Pilkington says, "If you have your lower animals to contend with, we have our lower classes." In the allegory of WWII, Mr. Pilkington represents England and Napoleon represents Stalin.
Stalin, leader of Communist Russia had the oppressed working class (proletariat) and England had its oppressed lower classes. This conversation begins the tenuous relationship between the pigs and the humans: it is an uneasy relationship based on power negotiations and mistrust. What the humans and pigs have in common is oppression of their people/animals.
As the animals outside noted, it was difficult to tell the difference between the pigs and humans. The implication is that neither England nor Stalinist Russia has a truly democratic and just system. The card game itself is a metaphor for the tenuous relationship that became the Cold War. The United States and England did not trust Russia and vice versa. Each suspected the other of cheating. But they became allies.
So, as an allegory, Animal Farm is not just an indictment of socialism (as it was horribly misapplied under Stalin). The novel was an indictment about oppression in general. After all, if the animals (lower classes) could tell no difference between the humans and the pigs, the inference is that neither system (capitalism nor communism) had as yet been utilized to help those lower classes.
The last few lines depict a room full of despots, each guilty of oppressing his own people, arguing over whose flawed system is better.