NOT IN vs IN Do Not Return Complimentary Results

Question

Hi I am working through example #7 from the sql zoo tutorial: SELECT within SELECT. In the following question

\"Find each country that belongs to a continent where all p

Answer 1

Because every other continent has at least one country with less then 25 Mio population. That is what this says.

  SELECT name, continent, population FROM world 
WHERE continent IN (
    SELECT continent FROM world
    WHERE population < 25000000)

Translating it into words: From the list of all countries (in table world) please find all countries where the continent has a country that has less than 25 Mio population.