It was probably less about the description and more about where she found both of them. She has a description about the guild master, but if she tried to find him on the street, it would probably be hard. But she went to the place where the guild master was known to be, and saw a man fitting the description, so she probably assumed.
As for the emperor, if she had met him at a throne room or smth, she probably would've assumed he was the emperor as well. But she met him on the street, buying flowers. And he subsequently just wanders around without an escort. Plus, he gave her an alternate name to attach to his face, so I don't see it as too far fetched to say she assumes he is just a really handsome extra.
Plus, since the emperor also wanted to use Sir Rojas as a double during the masquarade
I posted by accident, I was gonna say that since the emperor wanted to use him as a double, we can also assume they have a similar appearance, and probably even the same colored eyes.
That's my theory anyway!
So I’m confused how there was a detailed description of the guild master in the novel she read but not a description of the emperor who was supposed to be the novel’s male lead. If there was a description, she would’ve known that he was the emperor or at least suspected it ┑(￣Д ￣)┍