This is a dig at Scotland, which has been the butt of English humor for most of history. Oatmeal is a perfectly edible food, nutritious and healthy, but it was not popular in Swift's England. In Scotland, which was a much poorer land, oatmeal was a staple. Gulliver the Englishman, unable to eat hay and straw, had to content himself with the food of a Scotsman.
In 1755, Samuel Johnson (1709 - 1784) in his famous dictionary defined "oats" as a grain which, in England, was fed to horses and , in Scotland, was fed to men.