It's doubtful that the nurses (or anyone else) was infected at that point. First because it was not long after the family contact and they were not infected. Second because it is much more likely that the two nurses were infected when Duncan went acute and they had inadequate protection for that.
Actually, the CDC investigation found that Duncan was contagious, and the nurses were using PPE incorrectly when they arrived on Sept. 30. Frieden said CDC investigators found that on Sept. 30, the day of Duncan's Ebola diagnosis, healthcare workers were donning "three or four" layers of protective gloves and gowns, thinking that would increase protection from the virus. But such measures, Frieden warned, may have increased infection risk. There were many issues with the PPE, not just the one quoted here. I do not want to quote too much from the LA Times, but the full article has a lot more information.