It was a creative ruling in multiple senses of the word. They invented the idea from whole cloth and it effectively created new laws from the bench.
> The Court decided that Filburn's wheat-growing activities reduced the amount of wheat he would buy for animal feed on the open market, which is traded nationally, is thus interstate, and is therefore within the scope of the Commerce Clause. Although Filburn's relatively small amount of production of more wheat than he was allotted would not affect interstate commerce itself, the cumulative actions of thousands of other farmers like Filburn would become substantial. Therefore, the Court decided that the federal government could regulate Filburn's production.