It's not exactly the vehicle I would choose to take out on a trail like they show, but it's good to see that it can handle it. The percentage of GLEs that will ever see a trail like that? I imagine very, very few.
I think they did it as a test to see what it can do but I do agree with you on the fact that most GLEs will not see these trails in their lifetimes. The scratches made to the undercarriage sends shivers down my spine.
Dirty isn't the problem, it's the potential damage to the undercarriage. The writer also notes that the GLE makes for a more comfortable landing on bumps thanks to the sport seats and the front suspension. Most of us will probably appreciate it on speed bumps and the occasional pot holes.
You can probably buy a piece of self-adhesive moulding trim and stick it to the underside of your bumper or you can try adding on the invisible car bra. They should sufficiently protect the underside of your bumper but even if it does get scratched, most people won't see it.