According to the documentation, the chief playback audio session categories are:
AVAudioSessionCategoryAmbient
When
you use this category, audio from other apps mixes with your audio.
AVAudioSessionCategorySoloAmbient
By default, using this category implies that your app’s audio is
nonmixable—activating your session will interrupt any other audio
sessions which are also nonmixable.
AVAudioSessionCategoryPlayback
By default, using this category implies that your app’s audio is
nonmixable—activating your session will interrupt any other audio
sessions which are also nonmixable.
So, you've deliberately picked the only playback category which doesn't silence nonmixable background audio. If you do want to silence nonmixable background audio, pick one of the other categories. There's nothing wrong with changing categories at any time.