Thanks Alyssa, I figured it in detail after talking to support team. The wording is rather confusing to me so I had to talk to someone in person.
All Axure doc says yes multiple people can work on the same file (or the project) but in reality, I’ve realized that only one person can work on the page. As you said, only one person can check out the book at the library. Meaning, no multiple people can work on the same book(or the page) like how Google docs allow users to work together on the same doc at the same time. I am just writing this down for the sake of other people who may get confused by it.