I can believe a 10dB loss of signal strength due to corroded connections - you only had to listen to the FM radio on my old Vauxhall Cavalier when you wiggled the antenna wire.
Regarding the remaining 30dB, What IO540 actually said was a loss of dynamic range of 30dB, not an attenuation of the RF signal by this amount.
By this, I understand him to mean that the noise "floor" at the receiver comes up by that amount due to the interfering effects he mentioned and you get a signal to noise ratio of say 25dB rather than 55dB (or whatever). What you hear in your headset, for the same RF strength, is still intelligible, but has more background hiss.