Movies
Whew-hoo, look who knows so much. This man is only MOSTLY dead, see there's a difference between mostly dead, and all dead.
— Miracle Max ‧ The Princess Bride

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker insists that mostly dead and entirely dead are different conditions. Treating such an unlikely distinction as an obvious correction creates the joke. The mocking opening adds to the comic certainty, making the explanation sound both authoritative and absurd.
Context reviewed