How is this possible? 3 LEDs trick
The Solution Revealed!

Hackaday summarizes:


  it uses two AC power sources, diodes, and inductors. In contrast to every single guess about where the circuit is hidden, the majority of the build is inside the battery connector. [Fredzislaw] did some amazing work hiding a 74LV132 quad NAND Schmitt trigger inside the battery connector. The diodes were easily hidden on LEDs 1 and 3 with some red nail polish, but we’re amazed by the inductor built into the LED seen in the title pic.


And two shots from the video, showing the circuit diagram and how it's embedded in the battery connector: