Confused with understanding the text about high input impedance and coupled noise for an electret microphone
The (electrical) noise is picked up by the wires between the microphone capsule and the amplifier.  

When the amplifier is attached directly to the capsule, you have almost no wire length between the capsule and the amplifier.  It pretty much doesn't matter that it is a high impedance circuit.

Once you are past that stage, the low impedance output of the FET amplifier helps to minimize the noise picked up on the long wires from the FET amplifier to whatever is actually going to use the signal.



So, if you have a capsule without a built in FET amplifier, and you use long wires to connect it to your own amplifier, then you will pickup lots of noise because of the high impedance circuit.