I think that the surprise has more to do with there being undiscovered messages in the book. But I like the idea that DD has been forgotten. The biggest thing DD did was being the champion of order against the Storm of Souls; but that wasn't something many people knew about, and, if I recall correctly, most of them were dead or lived as hermits at the end of the story. In his other adventures, other people had a much bigger stake or more public role. Like Miranda vs King David, or Luna, saviour of the Orcs, helped by Pink Man, a renowed orkish hero.
Snout also only knew of him as someone who made gifts to the library, which is why his curiosity was piqued when he ran into some very odd info about him.
One would assume that Arudak, who is an orc and therefore aware of Luna, and also is studying Dominic Deegan's legacy, would know they were married. However, if DD also received a different name in Maltak (like his father, who was known as something like "Pink Man"). Maybe Arudak knows that Luna was married with Orcname, but records that Orcname was known in Callan as Dominic Deegan have been lost.
A relevant panel: https://www.dominic-deegan.com/comic/2705-20110606/