The word 'waterfall' does not appear in this paper, not even to describe the model that Royce is criticizing (and it would be a completely inappropriate and misleading metaphor for the model he is advocating.) I suspect the use of 'waterfall' as a model of software development is a retronym coined (possibly by Royce, elsewhere) for what was once the only model of systematic software development, once its shortcomings and the need for an alternative became apparent. It has persisted because it is a convenient straw man: it is easy to make almost any new model look good when you compare it to the waterfall model. We should have burned this straw man long ago: 45 years have gone by since Royce explained its shortcomings and proposed better alternatives, so by now, we really should be able to justify our latest methods with something more than just "better than waterfall".