laclean Maybe this unit had different firmware/manufacturing from the one NotebookCheck reviewed. NotebookCheck's oscilloscope method can detect >1Khz and <100Khz PWM.
Could be the lines you see are subpixel array differences between the camera and display. I get the same effect with my Macbook Pro filming with Canon T5i and zoom lens, despite being PWM free until below 50%, at which point it is 100Khz PWM according to Apple sources.