That's incorrect. There are three situations where you can plot to take an empire/kingdom title.
- You have a claim on the title
- You are a pretender to the title
- The title is not the holder's primary title, but of the same tier, and your primary title belong de jure to this title
An example of the last one would be if you play as a duke in England, you can plot to take the crown if it's held by the King of Norway.