The guaranteed one-directioness
The fiberoptics inside the data diode enables the concept of no leakage, since the fiberoptics in itself just can transfer in one direction. This is also part of the guaranteed one-directioness.
Since nothing may under any circumstance leak within the data diode from the secure (red) side to the not secured (black) side, the data diode is the one failsafe way of preventing leakage of data.
As the data diode separate the two sides (red and black) into two physically different PCB’s (Printed Circuit Boards) and only connect an optical fiber between the two sides even galvanic leakage is disabled.