This is not proof of duck-ness, merely a simple example of inference. This phrase is used as a metaphor in conversation or journalism to describe the various behaviors of people, companies, and lawyers. I learned the phrase in my Catholic high school in a class on Christian morality, although the exact context in which it applied escapes me. (If behavior A seems immoral, it probably is(?)...)

By extension, If it looks like a duck,
and it sounds like a duck,
and it acts like a duck,
people will most likely hold duckish expectations of it, and treat it as a duck.