Not a native German speaker (One side of my family is German so I have assimilated some of the culture, but I know only small amounts of german), but I would ha

Not a native German speaker (One side of my family is German so I have assimilated some of the culture, but I know only small amounts of german), but I would hazard a guess that an expanded form of it is Don't worry, [no matter how great they are,] they also cook with water meaning something along the lines of "No matter how great they are, they are still human in the end".