It's a bit of a confused claim.
First, what is a "molecule"?
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge. However, in quantum physics, organic chemistry, and biochemistry, the term molecule is often used less strictly and applied to polyatomic ions.
Note: We are talking organic chemistry here. Basically, a group of atoms bonded together.
Now, what is "plastic"?
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs. Monomers of plastic are either natural or synthetic organic compounds.
So, there isn't one single type of plastic - and typically they are made of huge molecules. My high school chemistry teacher claimed a plastic ice-cream container was in fact a single molecule, due to all the cross-linked bonds between the long chains of atoms.
[UPDATE: This claim has been challenged. See related question. This undermines the forcefulness of the following argument, but doesn't affect its validity.]
So, we can say "Water is one molecule away from being a plastic ice-cream container." or "A vacuum is one molecule away from being a plastic ice-cream container."
Given that, "Margarine is only one molecule away from being plastic" is a confused claim that is both true and does not tell us anything about margarine.