I finally answered my question from all these years ago when I purchased this DVD from eBay a few weeks ago.
Yes, the 5.1 surround content on this DVD set is very limited. However, some of it is a real gem!
First off, the "High Road" content (4 songs filmed during a 1982 concert in Frejus) must either be an upmix or just a very ambient surround mix, cause there's not much going on in the surround channels for these tracks at all, but the performances are great!
Then, we get to the real gems of this DVD set!
The DVD includes the music videos from the "Avalon" album ("More Than This", "The Main Thing", and "Avalon") all in discrete 5.1 audio!
These are the same mixes that feature on the long-OOP "Avalon" Multichannel SACD, so if you don't have an SACD player or you don't want to spend the money to get the SACD, check out this DVD set to get a real good taste of how good the "Avalon" 5.1 mix is!
https://www.discogs.com/Roxy-Music-The-Thrill-Of-It-All/master/470168