The windmill was, in fact, Napoleon's own creation
Comrade Napoleon...[is] already making arrangements to send Boxer to be treated in the hospital
Lies & Deceit
In this scene, Squealer is explaining to the other animals how the windmill was truly Napoleon's idea all along. That is a lie, and Napoleon was able to get away with this due to him running Snowball, the real pig who came up with the idea of the windmill, out of the farm. Squealer is lying to the animals in order to make Napoleon more loved and seem smarter.
With Boxer's injured hoof needing professional help, the pigs exclaimed that they found a hospital to take care of Boxer. That lie was not debunked until Benjamin read to the farm what was written on the side of the van: "Alfred Simmonds, Horse Slaughterer." The pigs lied to the farm again which led to the animals believing that the pigs wanted to help Boxer when in reality, they used his death to show the animals how much work the animals should be doing on the farm.