I don't think you're entirely correct here. The regulator will create noise that has a fundamental frequency of 500 kHz. That we agree on. However, this 50 MHz noise can most certainly be caused by the regulator. It's critical to note that it's not a constant 50 MHz noise. It's clearly attenuating quickly and occurring sharply. If the author had zoomed out to the 500 kHz time scale, I'd bet we'd see this noise every time the switches change.