Exactly. People may not realize they are sick right away. It's not like the moment your temperature goes to 98.7 you think to yourself, "Uh oh. I'm starting to get sick."
Yes, but the ebolavirus needs to be "carried" out the body -- namely by body fluids.
If the second infected nurse were coughing and sneezing all flight long (and she may have been, not yet known), that would be a much bigger issue. From what we know now, she was
externally asymptomatic on the flight (allowing for the very beginnings of a fever to have been present).