Considering in the real war, there were no battles south of the Sahara there would have never been a reason for a fighter to launch an attack across the Sahara from North Africa. So based on any historical fact of a fighter never crossing it during the war as reason to denounce the idea of it being reasonable that it could in fact cross in to me unreasonable.
Also, considering the advances fighters made in WW2, the fact that it had never been done is a kin to rationalzing that 1.) Japan ships cannot cross into the Mediterranean to help Germany, and 2.) A bomber once it gets longe range aircraft, cannot use that extra space because it has never been able to do it in the past.
I am not an expert in the technical abilities of the aircraft at the time. I do understand however that German troops could not pass through the Sahara without probabbly dying and vehicles would likely breakdown if they tried.
But whether or not a fighter had the ability to fly over it safely, considering how high above they ground they are, is a perfectly rational question. With the basic understanding, (not and expert one) of the development of aircraft, from the first “Lindy Hop” across the atlantic in the 20’s, to the solo flights of Amelia Earhart, to the advances of longrange aircraft by the end of the war, it is not clear to me at all why fighters could not “obviously” fly over the Sahara.