"They went out without an umbrella!"
Translation:Elles sont sorties sans parapluie !
No difference in duration. Sortir means you leave to do something outside the home. Partir just means you leave. You could return later after either verb (or not). An easier way to think if it is use sortir if you could say "go out" and use partir if you just mean "leave."
I would guess that while "Sont allés" is "went" its a looser translation than either "sont partis" ("left" or "parted") or "sont sortis" ("went out" / "exited".)
Also, the section this particular question is associated with is focussed on learning the latter two so your answer being included may just not have been considered. (That may not be easy to account for if your getting general quizzes across all sections though.)
Ils sont sortis was my answer and it was flagged as wrong. Elles sont sorties seems to be the only correct version. Is that equal rights according to DuoLingo? Or is it me being politically incorrect or having the gall to ask that? Or is Elles sont sorties the only correct answer and I am just unaware of that...?