I admit that it is slightly ambiguous, but to me "the test cases ensure" reads like you as the developer can assume that the test case input will always already be sanitized and you don't have to worry about data anomalies.

This needs more sample test cases.. Yes I get its a fairly simple question, but even two more test cases could build confidence

The description states "Please keep in mind that the test cases ensure that the number of people in the bus is always >= 0. So the return integer can't be negative." however there doesn't appear to be a specicic test case that checks for this. To me this states that the developer needs to check that their return can't be negative, not that the developer doesn't need worry about negative numbers, because you can't have negative people.

I would either clarify, or add a test to ensure that data anomolies are properly handeled by the developer.