Comic reader here, but this is tangential information - he used to write fiction books, then later transitioned to travel books because they sold better. When Debbie’s doing house cleaning towards the end of the episode, we see her take out Nolan’s books, including ones with very sci-fi-eque covers and titles

I mean it’s the Viltrumite Empire, their range of control is massive, and as we see with the Coalition of Planets they have spies across the galaxy. The Thraxans are a spacefaring civilization, so it stands to reason that some trader or explorer or whatnot mentioned the super powerful mustachioed menace on another planet and word eventually got back to the Viltrumites

The funny thing is that this is fairly soon after >!Mark’s trip to the future where he just casually rips off the Immortal’s head!<, so on my first read-through when Mark grabbed his face I was like “oh no”