Snowball was in league with Jones from the very start! He was Jone's secret agent all the time!
I do not believe that! Snowball fought bravely at the battle of Cowshed. I saw him myself. Did we not give him "Animal Hero, First Class" immediately afterwards?
Now when Squealer described the scene so graphically, it seemed to the animals that they did remember it.
And do you not remember, too, that it was just at that moment, when panic was spreading and all seemed lost, that Comrade Napoleon sprang forward with the cry of "Death to Humanity!" and sank his teeth into Jones's leg? Surely you remember that comrades.
In this panel, Squealer begins the Dramatic Irony by lying to the animals to make them believe that Snowball, who was Napoleon's greatest political rival when he was around the farm, was actually a traitor. The majority of the animals believe him without a doubt because of his "superior intelligence", and only the readers know the truth.
Boxer attempts to deny the fact that Snowball betrayed the farm, and he isn't wrong. It's what we as the readers believe and it acts as an opposing opinion to Squealer, who can usually persuade them without question. This is the part of Dramatic Irony where the characters almost catch on, but are fooled into believing otherwise.
In the final panel, Squealer is able to use is master-manipulator skills to persuade the animals' memories into believing that it was Napoleon who made the greatest contribution during the battle of Cowshed and not Snowball. The reader knows this isnt' true, but the animals are easily tricked into thinking otherwise.