Why is it ironic that the pigs carry whips in Chapter 10 of Animal Farm?

One of the primary rules of the animal rebellion was that
those who walked on four legs were good, and those that walked on two legs were bad. In
Chapter 10, the animals were astonished to see the pigs emerge from the farmhouse
walking upright on two legs. Additionally, Napoleon now carried a whip. This irony is
important because Mr. Jones and his workers used to use whips on the other animals--one
of the abuses the animals hated most. Napoleon had already broken another of Old Major's
doctrines--that "no animal must ever tyrannise over his own kind"--but now he was
completing his transition into pseudo-humankind: He had become the animal version of Mr.
Jones, ready to punish with a whip those animals who did not obey his
orders.