I think the discussion here indicates that there needs to be more explanation regarding these 3 verbs in the lesson above. As far as I can work out their use depends as much on syntax as semantics. My notes eventually say Quitter is the only transitive verb and must have a direct object, Partir and Sortir are both intransitive, but Partir is more to get out (i.e. leave/escape) whereas Sortir is to go out and only means leave if it is used with de . Partir can be used without any object at all, I'm still not clear if Sortir requires an indirect object or can be used without.
(edit)... So I thought I had eventually cracked this, then 2 minutes after writing the above I get marked down my answer "Charles sort pour Londres" for How would you say "Charles is leaving for London." ? Apparently the answer is Partir, I am afraid the lesson fails make any clear distinction between Sortir and Partir.