Perhaps, in this particular case, this French family didn't read English?

It's hard for me to personally relate to something like this. I mean, I do plenty of stupid things and have doubtless survived my own stupidity many times without ever even realizing how stupid I was. But surely the concept of a desert in high Summer, complete with sand dunes, ought not be so unfamiliar as to make the hazards anything other than glaringly obvious.

As for rental cars, I plead guilty to knowing of some nice camping sites that I jokingly say are accessible only by 4WD OR rental car, meaning that your car is at greater risk of getting beat up or stuck, but can probably make it. But none are so bad, remote or hazardous that you couldn't walk out in a few hours or have a buddy with a pick-up pull you out of a rough bit easily enough.