Why can playsInSilentMode not be false when mixing is set to "duck others"?

Please provide the following:

  1. SDK Version: 40
  2. Platforms(Android/iOS/web/all): iOS

Line 191 of EXAV.m says:

if (!playsInSilentMode && interruptionMode == EXAudioInterruptionModeDuckOthers) {
return UMErrorWithMessage(@“Impossible audio mode: playsInSilentMode == false and duckOthers == true cannot be set on iOS.”);

Why is it forbidden to mix these? Is that documented somewhere in the Apple docs? Did not find anything.