A considerable amount of the text in The Widow's Son consists of the footnotes, which often cite existing books but which also cite a number of nonexistent ones, such as Golden Hours by de Selby, the time traveling sage, and his intellectual colleagues and rivals. (See this post.)
The story in the footnotes about de Selby and his controversies mostly exists outside of the universe of the main novel — except in Chapter 22 of Part Three, where Sigismundo has an encounter with a man who appears to be de Selby.
It's not the only moment of apparent time travel in the Historical Illuminatus novels. At the very beginning of the first book, Sigismundo is "lost in the forest with a Red Indian, seeking the supreme Wakan," something that apparently cannot actually happen until the third book.