Michael Dutton has made new quad mixes for a number of vintage recordings, such as the Lt. Kije that accompanied Alexander Nevsky. And of course he also does quad mixes for all of his new classical recordings.Michael Dutton doesn’t mix these. They are old quad mixes.
The Sonata is added as a “bonus work” in stereo only. Presumably because it was never mixed to quad back in the day.
A sonata in quad might not be much to hear anyway.
And yeah, I'll admit a solo piano only needs so many channels. But there is benefit to ambient quad - give more of the hall sound and the "you are there" feeling. And stereo famously doesn't serve the piano well - the famous "20 foot wide piano" problem. Additional channels are a way to mitigate that.