Yet one more failing of the IDE - it should only take note of keywords for libraries you actually have included.
I think that it is more a failing of the Stepper library author. Keywords are supposed to be just that. Key words that mean something to the class, like method names. The words direction and speed are not methods of the Stepper library, while setSpeed and step are. It is reasonable that setSpeed and step are in the keywords file. It is not reasonable that speed and direction are.
The whole color-coding stuff is silly, though. The word step should only be colored if it is a method(). It is not rocket science to recognize .step( and ->step( as distinct from // We need to step until...