Squealer's assertion that Snowball's claims of heroism are "much exaggerated" foreshadows the way Snowball will be demonized throughout the rest of the novel and used as a scapegoat—or scape pig—every time something goes wrong at Animal Farm. Truth has become elastic and will now change almost moment by moment to agree with whatever suits the interests of Napoleon and the other pigs.
Further, it foreshadows how the pigs will use fear to control and manipulate the other animals. Right after he makes his statement (or, more accurately, tells his lie) about Snowball's overrated heroism, Squealer states:
Discipline, comrades, iron discipline! That is the watchword for today. One false step, and our enemies would be upon us. Surely, comrades, you do not want Jones back?"
Squealer diverts attention from Snowball before the truth about his heroism can be examined by encouraging fear of enemies such as Jones. This kind of propaganda will make it increasingly difficult for the other animals to discern truth from falsehood. It is part of the death knell of their dreams of a utopian society based on equality and mutual respect.
In Chapter Five, Squealer claims that Snowball's heroism is "much exaggerated." Squealer does this to blacken Snowball's reputation after he is driven from the farm by Napoleon, but it is also significant because it foreshadows a number of other events in the story.
First of all, it foreshadows the blaming of Snowball for the destruction of the windmill in Chapter Six. Even though the windmill is destroyed by bad weather, Napoleon instantly blames Snowball and uses this as an opportunity to sentence him to death.
Secondly, it foreshadows the murder of the pigs and the hens in Chapter Eight. In order to justify these extreme acts of violence, for instance, Napoleon accuses these animals of being Snowball's "secret agents," a move which further cements Napoleon's dominance while ensuring that the animals are too afraid to defy him.
The animals saw Snowball's heroism for themselves. However, after Snowball is expelled, Squealer tries to tell them that Snowball was really not a hero afterall. This foreshadows the rewriting of other history that will take place during the rest of the story. It also foreshadows the rewriting of the original commandments given written by Snowball and Napoleon. Once Snowball is gone, Napoleon is free to rewrite both the commandments and history according to his point of view. This parallels the rewriting of history that took place during the rule of Stalin in the Soviet Union.