Alastor did not reveal his identity to the humans when he acted as a mediator between them and Ames. Hiding his identity was not something he and Ames agreed on, but he was sensible enough to know that revealing his identity to the humans might cause some problems.
He was not worried about the Fates trying to do something to remedy the Human plane's knowledge of the Lower Realm. Because, like Nolan, he was aware that one of the Fates suffered an injury from Kyran. Thus, they could not interfere with anything in the Lower Realm that could change the course of the future.
Moreover, Alastor knew that the Human plane was one of those special planes in the Lower Realm that must not be destroyed during the War of the Sovereigns. Even if the Fates intervened with the events happening in the Human plane, they could not overdo it because, unlike in other planes, their influence there was only at a bare minimum.
