how does it improve EFT immunity
I need to say this first: -
An external reset button (as stated in the picture in the question) means a button that is connected to the equipment through a port on the box of the equipment that is being tested. An internal button on the PCB of the equipment is not subject to EFT testing.
In other words this only applies to an external button as per this: -

The repetitive EFT test consists of a number of fast transients
coupled into power supply, control, and signal ports of electrical and
electronic equipment.

Above quote taken from here. You don't need to access the whole document; just the quote above. In effect an external reset port is regarded as a control port and may be subject to testing.
So, a reset button wired to a piece of equipment may be subject to EFT and, the capacitor soaks up the energy of the EFT and largely prevents a reset. In other words, the capacitor acts like a low pass filter and significantly attenuates high frequency (EFT) noise. This prevents false external resets.

in fact the 100pF is low impedance at high frequencies so its shorting
reset pin to VDD ?

That's one way of putting it - it acts like a short to the EFT and, keeps the reset pin close to the same voltage as \$V_{DD}\$.