This is a neat example of the flexibility of English - and how slight changes in meaning come from the contexts.
With reference to "the main idea", see here for usages which should indicate meaning:
[Davies/BYU] BYU-BNC: British National Corpus
1. What is the main idea of the passage? Fundamentally, what is the theme/topic of the passage.
2. What is the passage mainly about? Fundamentally, what is this passage discussing.
3. What is the writer's purpose for this passage? What does the author mean in this passage - what are his/her intentions?
4. The author mainly wants to tell us _______. The author is giving us these facts.
Student or Learner