St. Thomas held that something must be ingested per modum cibi ... in the manner of food. So, for instance, if you accidentally swallow a bit of mouthwash or some food that gets dislodged form your teeth, etc. that's not considered "eating". So it's based on a common sense understanding of the human activity called "eating". Swallowing a bit of toothpase is NOT eating in the commonly-understood meaning of the term. Church law is based on common sense.