Luo Xiao was aware that Huo Yu was a new person. Although he did not know this young man very well, he still had great expectations from him.
However, even though Luo Xiao was very optimistic about this young man, he was not stupid. The Heavenly Oath he required had to be completely fool-proof, and thus he informed the young man about his real name.
If Hou Yu were to later find out that he was not Gu Tianyu, then the Heavenly Oath would be useless, as he had sworn to the wrong name, and if he merely used the word 'you' it could also be debated who 'you' actually was.
At the same time, Luo Xiao also knew that there was not much risk in admitting his identity to a servant in the house. If this young man was hesitant, then it would not be a big deal, as both Ye Feng and Ye Chao were present. They would be able to end his life right away to remove any threats for Luo Xiao.