How is the quote from Huck Finn "Give a nigger an inch and he'll take an ell" an example of Twain's satire exposing hypocrisy?

Huck quotes this "old Saying" when he hears Jim start to speak about what he'd do with his freedom--buy and therefore free his wife and children.  Huck becomes alarmed at this because it undermines the social structure -- Jim is supposed to stay in place-- even though both Jim and Huck are outcasts to that structure.  In helping Jim get free, Huck sees that his actions have potentially allowed a few more slaves to get free -- and he sums up the situation with the quote.  However, the original quote was probably something like "Give an inch, and they take a mile."  But Jim isn't planning a full-scale slave revolt; he merely proposes to free his family, a much less ambitious task.  Therefore, Twain has Huck use the word "ell" (an old measure, about 45 inches) and the reader's not sure if Huck says that out of ignorance of the original "Old Saying" or if he's appropriately sizing the quote to Jim's ambitions.