About the impassible part as far as ground units go, I imagine the historical pass did not cross the Sahara due to its terrain being to hard to fight in.
With my experience in Iraq, I can say that vehicle maintenance and weapon maintenance was horrific. As long as we cleaned out air filters and cleaned our weapons daily it did not hamper the mission. Of course if we ever had to do battle just after a sand storm, who knows. I was part of the second wave and spoke to soldiers who indeed had to fight in sand storms. It was difficult.
I can only imagine the Sahara being much worse and vehicles being more prone to breaking down in those days, so I imagine both sides just decided not to fight in the Sahara or try to out flank each other by traveling through it.