I also found and applied this to my Soundtouch 20 and both Soundtouch 300s.
It appears to be a very small update but as you said, no information on exactly what it does and no offical posting as there normally is.
Afterwards I did some testing to make sure I found nothing wrong - I tested Spotify (with and without Play all), Netflix, from Mac via HDMI in (Hbo Go, ITunes Movie playback) and ITunes music playback from My Cloud NAS - in general no obvious changes however my wife claims there was an 'echo' on Netflix 4K TV playback to one of the ST300s but I was not sure myself.
