I've taken the Streetmachine off-road on various occasions. Being short of leg, I'm used to sitting out of the seat when stopping, so riding along like that when it's useful to be able to shift weight isn't a problem.
The limiting factor is nerves (basically, you need to be sure you aren't going to have to dab a foot down, so you either fully commit or chicken out), followed by what you can achieve with a small diameter front wheel, 40mm Marathons, and no ability to bunny hop. As it's a suspended bike with wide tyres and disc brakes, I'm better equipped for slippery rough stuff than most forumites' audax/touring bikes, and will happily ride when most people decide to walk. That pushing a USS recumbent is so unwieldy has a lot to do with this.
It's also the only bike I've repeatedly managed to recover from front-wheel skids on, and that includes proper mountain bikes (not sure if that's down to my skill level or a magic property of the bike).
I recall Ben of Kinetics built up a Streetmachine frame as a single-speed off-road bike (with appropriate tyres) and found it surprisingly good. Can't find that write-up now, unfortunately.
Don't take this video as an example of what a recumbent can't do. The rider is clearly inexperienced, evidently with the bike, and possibly with the route and/or singletrack riding in general. They're also not leaving enough room behind the bike in front to compensate for the stoppy riding style of upright riders off-road, causing them to get stuck in places that they'd be able to ride if they carried some momentum.
But yes, it is fundamentally silly. Expedition touring 'bents like the Streetmachine and ICE Adventure are designed to work well on rough stuff, so that you won't get thwarted by poor quality 'roads'. That's perfectly sensible. Singletrack, not so much. Using a recumbent for more gratuitous off-roading is a silly idea, unless the rider has a disability, or is trying not to be blown over by Antarctic wind or something, and probably even then.
