The API classes are not available as Ruby files so we do indeed not have any possibility (and need) to require them. And the little what the sketchup.rb actually contains is often not needed. The comment line is certainly not precisely formulated.
But SketchUp Ruby scripts/plugins/extensions still require a special embedded Ruby that provides these API classes. If one attempts to run them outside, they should fail (at load time and not somewhen late at runtime).
The requirement is probably meant as marker statement and not a litteral requirement of the little bits that this file currently contains.
How else would you suggest to “mark” scripts so that they need a special environment? Also if the Ruby API was implemented as a sketchup.rb that loads a C extension, would it be better? I sometimes drop in alternative SU API implementations (mocks) for ease of testing in the system ruby interpreter, where a sketchup.rb requirement makes sense again.