If just the rats were made intelligent by NIMH experimentation then why can every other animal on the farm talk and read and work machinery and stuff too?
The cat is obviously unintelligent, so if the rats brought back like an intelligence "disease" or something, the cat somehow wasn't affected...
And anyway, they could all talk before the experimentation anyway so the experimentation didn't even DO anything except extend their lifespans and help them learn mechanics and electricity.

The magic was just a "given", I mean like I take it that magic simply exists in this universe, I don't need an explanation for that.