How do 100mA ELCBs provide human protection?
ELCB were not initially quite same than a RCD, but now the terms are used interchangeably. ELCB is a device to detect leakage in a high impedance ground system by measuring ground voltage, so it does not detect current difference between live and neutral which is how the RCD works.

The point of a 100mA or 250mA protection device is not so much to protect humans touching mains electricity directly, but to protect devices and buildings from fires caused by improperly installed or damaged wiring so that they do not cause fires.

Also 250mA leakage to ground within a device like a motor may not pose a direct hazard to humans at all, unless the ground wiring is faulty and has too much resistance so that the 250mA causes a dangerous voltage to grounded metal casing, when compared to other grounded devices.