I don’t think you can draw that conclusion so bluntly. There are good reasons why a “purely visual overhaul” would have WWDC keynote deadlines. For instance, Developers want to know about visual changes too, because they want to start reworking the visuals of their own apps to fit in with the new style of the OS.
More importantly, although WWDC is a developer conference, the keynote is aimed firmly at the media. Apple knows that if they want positive PR coverage on that day, having the visual overhaul ready for demo is essential.