IF the switches are simple series SPNO switches as shown and
IF the LEDs are standard LEDs then by definition it is not possible - he is using video fakery and the demo is less than valueless.
"Not possible" is able to be said with certainty because examining even SW1 + SW2 + SW3 in series current can flow only when all 3 are operated.
HOWEVER if the "switches" and "LEDs" are modified then it is possible and it becomesa conjuring trick with rigged props. Still pretty pointless, but possible.
One of various ways:
Each switch contains a current determining circuit.
Each LED contains a window comparator and current bypass circuitry
LED1 lights for I1 or (I1 + I2) or (I1 + I3) or (I1 + I2 + I3)