Both interfaces give access to $XDG_RUNTIME_DIR/pulse/native as pulseaudio does.
At present the only thing audio-playback doesn’t give you is the ability to open a recording stream, with that restriction implemented on the Pulse Audio side (at least on Ubuntu). If you could reconfigure PA via the pulseaudio interface, then it is likely possible via audio-playback too.
With that said: the obvious question is whether this is desirable: I’m not sure whether a snap should expect to be able to reconfigure PA via any of pulseaudio, audio-playback, or audio-record. Blocking such access would probably require expanding the hooks patch set further though. @jdstrand: do you have any thoughts on this?