You may be correct about the early treatment with anti-virals, and antibodies may help, too (from plasma).
There is the possibility that the timeline is either being messed with to keep the narrative intact in Duncan's case, or, even more likely, that the entire infection to symptomatic timeline is dependent on initial viral load and means of infection. (IOW, not so hard and fast as we have been led to believe.)
That opens another kettle of worms, especially when it comes to declaring someone "Ebola free" and safe to live a normal life. Longer quarantines may be indicated, both prior to declaring someone 'clean' and after infection as well.