Warhammer Fantasy is a fictional universe in which a bunch of tabletop games, rpgs and video games are set. It has normal fantasy staples like orks and goblins, dwarfs, elves and so on. It also has a race of human sized rat people called Skaven. The Skaven mainly live underground and have tunnels basically everywhere.
The main human faction is called the Empire and is mainly inspired by the Holy Roman Empire in our real history. Nuln, Marienburg and Altdorf are major cities in this Empire. Of course, the Skaven have a presence there, too. But the average human citizen of the Empire doesn't know anything about the Skaven, since they prefer to remain hidden. There are rumours about the existence of such ratkin, but the commonly accepted reality is that they don't exist.
Hence it is humorous to put conspiracy theorist Alex Jones into this setting to talk about it.