FM immunity relates (in reality) solely to the bottom end of the VHF NAV band, and solely to receivers.
It all happened when the Government sold off the "guard band" - the gap in the frequency allocation between FM broadcasting and aircraft navigation. So the FM broadcasters now come right up to the edge by 108MHz. If you're flying an ILS on 108.notalot, your receiver will have a hard job coping with Radio Free Toytown as you fly your approach.
So we all have to buy new radios to fly IFR in most controlled airspace. I blame the Government (but then. I often do - they're supposed to be in charge).
There was an early theory (now mostly disproved) that the problem of FM interference could be solved with filters rather than new radios. Those of us who know a bit about radio were sceptical - how do you make a filter with a very sharp cutoff between 107.95 and 108.00, and yet broad enough to go all the way to 118 or even 136 MHz...