The metaphor states that the person is downing  (i.e., typically connotes eating or consuming something quickly) a full pound of meat during a single meal, which contradicts in the another statement that it is impossible to do.
Consider the staetement "It is impossible for a single person to down a pound of meat in the course of one meal.". Based on this statement, is "Some people can down a pound of meat in the course of one meal." a true statement? Yes/no/unknown