Ok, then the answer is "they do use a slightly different version of the tool than what you learnt at school. That version is more rigorous (which is why they like it) and in that version 1 is neither a prime nor a composite. In the version you learnt at school you're told that 1 is not a prime and you can't really say that it's composite either, but no clear explanation of why it is not a prime; in the version that mathematicians use there's a name for what 1 is, and a rule to find things like it in other number systems."