Why would someone use the power of their domain to write out a story about a phoenix? Maybe there was more to the story than what he could see at a first glance, Xuan Hao did not understand it too clearly, but decided to take the book with him and investigate it close when he returned back to the sect.
No matter what, the power of a domain left in the book could help him gain a deeper understanding towards how a domain worked and increase his own comprehension towards it. The power of a domain left in the book was like a guiding light showing him how a fully-fledged domain would look like.
Even if he did not manage to figure out what the meaning behind the book was, he would still be able to gain a lot from studying the power of a domain left behind in it.
Will this be the last we see of the supreme flower breaking mantra...?