The first edition of the rules had Sardinia as part of that calculation, but it was removed in the second edition. Looks like the example was not updated to match.
Thanks very much for this - that makes sense from a rules point of view.
From a realism perspective, you can see that aircraft in Sardinia could attack the supply route, but once Sicily has fallen to the Allies the writing is on the wall and the aircraft would probably be withdrawn to defend the mainland. (I'm considering abstracted aircraft here, of course.)
Third Reich had somewhat similar rules for Italian surrender, but they allowed Allied control of either Sicily or both Sardinia and Corsica to be one of the factors.