The rumor itself. One of the main points of making a "PRO" version of the console is that Sony can take advantage of technology that comes out years after the OG system is released PLUS sell it at a high cost so that you can see and feel the difference between the two consoles.
A $600 PS4 at launch wouldn't have been the PS4 Pro we have today. It would have been a worse product. And it would have cost $200 more than the current version of the PS4 Pro also. There's very little upside to selling a PRO model at launch, "compared" to waiting 2 or 3 years.