For that needs of the application note, the resistor sounds is limited to thermal (Johnson) noise. To maintain a low degree of this type of noise, resistance values must be as lower as possible due to the fact RMS voltage of thermal (Johnson) sound is proportional to your square root https://low-noise-amplifier93578.blogunteer.com/33731314/the-smart-trick-of-pin-diode-switch-that-nobody-is-discussing