How individual LEDs in LED strips (or matrixes or cubes) are being powered when supplied with >=12V instead of 5V?
The WS28xx series leds have an ic in them to do the PWM and the protocol. Therefore it is not unreasonable that the 12V ones (and the 5V ones for that matter) manipulate the PWM such that the LEDs themselves don't get damaged. One thing to note with LEDs is that the current has to be limited, not the voltage but the voltage has to be sufficient in order to light the LED (which varies based on the actual LED and the color).