I can't tell from the listing whether it contains just a two-channel mix or includes a multi-channel one as well. If the latter, could be that the engineer went back to the original multi-track masters, from which Vox made the original quadraphonic recordings on LP.
If it contains just a two-channel mix, it might be QS-encoded, but it's unlikely.
The Mussorgsky recordings first appeared on the quadraphonic Turnabout LP, QTV-S 34633 and later on one of the earliest Moss Music Group Vox Cum Laude CDs, MCD-10014. I was still buying new stock for my
MultiChannelSound Shop when the CD appeared. My contact at Moss confirmed that this CD retained the QS encoding.
If the DVD used the same master, then you are in luck.
Unfortunately, the LP quad mixes for both the Mussorgsky and Holst recordings provide only ambience from the rear channels. Therefore, it's very hard to tell whether the surround effects are decoded or synthesized. In the case of the Mussorgsky CD, I had to take my contact's word for it.
In general, the digital descendants of quad recordings on Vox Box, Candide and Turnabout have lost their encoding. A couple of the recordings actually contained direct sounds from the rear. On Turnabout QTV-S 34594, an alarm bell sounds from the rear in "Hurricane" from "Catfish Row". Vox Box QSVBX-5132, Gershwin: All the Works for Piano & Orchestra also contains this. On my 1990 Vox Box CD (CDX-5007), the bell comes from the front.
Same is true of Ravel's Daphnis et Chloe Suites on Turnabout QTV-S 34603 and Vox Box QSVBX-5133, Ravel: All the Works for Orchestra. In Suite No.2, the female chorus appears in the rear channels. (I learned recently that this is as specified by Ravel.) I picked up a 1991 Vox Box CD of the recording, CDX-5032 to learn if it provided this effect. Not surprisingly, the chorus comes from the front.
If you get your hands on a DVD-Audio release of one of these, you'll be able to tell with some certainty whether the two-channel mix contains the QS encoding.
Unfortunately, unless you have specific information to the contrary, you should assume that CD and DVD-Audio releases of these old quad recordings are now stereo.
Larry